Freeze when converting to 'dynamic'

[zozorg]

Synfig version & platform:
1.3.11

Issue description:
Converting a vector to "dynamic" freezes synfig. Did not happen in version 1.3.10.

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[rodolforg]

It's a deadlock issue:

(backtrace from master branch)

#3 0x0000555555ed19a0 in std::mutex::lock() (this=0x555557c876b0) at /usr/include/c++/9/bits/std_mutex.h:100
#4 0x0000555555ed35a4 in std::lock_guardstd::mutex::lock_guard(std::mutex&) (this=0x7fffffff9808, __m=...) at /usr/include/c++/9/bits/std_mutex.h:159
#5 0x0000555555fcf285 in studio::Renderer_Canvas::clear_render() (this=0x555557c87630) at /home/rodolfo/projetos/synfig/synfig-studio/src/gui/workarearenderer/renderer_canvas.cpp:642
#6 0x000055555614e7d1 in studio::WorkArea::queue_render(bool) (this=0x555557db6580, refresh=true) at /home/rodolfo/projetos/synfig/synfig-studio/src/gui/workarea.cpp:2100
#7 0x000055555605c779 in studio::CanvasView::on_dirty_preview() (this=0x555557984f60) at /home/rodolfo/projetos/synfig/synfig-studio/src/gui/canvasview.cpp:2693
#8 0x000055555607caa5 in sigc::bound_mem_functor0<void, studio::CanvasView>::operator()() const (this=0x555557f9b9e8) at /usr/include/sigc++-2.0/sigc++/functors/mem_fun.h:1991
#9 0x0000555556078e64 in sigc::adaptor_functor<sigc::bound_mem_functor0<void, studio::CanvasView> >::operator()() const (this=0x555557f9b9e0) at /usr/include/sigc++-2.0/sigc++/adaptors/adaptor_trait.h:256
#10 0x000055555607417d in sigc::internal::slot_call<sigc::bound_mem_functor0<void, studio::CanvasView>, void>::call_it(sigc::internal::slot_rep*) (rep=0x555557f9b9b0)
at /usr/include/sigc++-2.0/sigc++/functors/slot.h:483
#11 0x0000555555cb83c2 in sigc::internal::signal_emit0<void, sigc::nil>::emit(sigc::internal::signal_impl*) (impl=0x555557ff53e0) at /usr/include/sigc++-2.0/sigc++/signal.h:798
#12 0x0000555555cb9440 in sigc::signal0<void, sigc::nil>::emit() const (this=0x555557982748) at /usr/include/sigc++-2.0/sigc++/signal.h:2804
#13 0x0000555555cb8c90 in sigc::signal0<void, sigc::nil>::operator()() const (this=0x555557982748) at /usr/include/sigc++-2.0/sigc++/signal.h:2820
#14 0x00007ffff71fac07 in synfig::Node::on_changed() (this=0x555557982670) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:336
#15 0x00007ffff71666b4 in synfig::Canvas::on_changed() (this=0x555557982670) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/canvas.cpp:111
#16 0x00007ffff71fa284 in synfig::Node::changed() (this=0x555557982670) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:207
#17 0x00007ffff71fad04 in synfig::Node::on_child_changed(synfig::Node const*) (this=0x555557982670, x=0x555556505000) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:349
#18 0x00007ffff71fa2b6 in synfig::Node::child_changed(synfig::Node const*) (this=0x555557982670, x=0x555556505000) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:213
#19 0x00007ffff71fac7d in synfig::Node::on_changed() (this=0x555556505000) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:341
#20 0x00007ffff71a6a4f in synfig::Layer::on_changed() (this=0x555556505000) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/layer.cpp:389
#21 0x00007ffff71fa284 in synfig::Node::changed() (this=0x555556505000) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:207
#22 0x00007ffff71fad04 in synfig::Node::on_child_changed(synfig::Node const*) (this=0x555556505000, x=0x55555653fdc0) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:349
#23 0x00007ffff71a6a75 in synfig::Layer::on_child_changed(synfig::Node const*) (this=0x555556505000, x=0x55555653fdc0) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/layer.cpp:395
#24 0x00007ffff71fa2b6 in synfig::Node::child_changed(synfig::Node const*) (this=0x555556505000, x=0x55555653fdc0) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:213
#25 0x00007ffff71fac7d in synfig::Node::on_changed() (this=0x55555653fdc0) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:341
#26 0x00007ffff722eb4b in synfig::ValueNode::on_changed() (this=0x55555653fdc0) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/valuenode.cpp:149
#27 0x00007ffff71fa284 in synfig::Node::changed() (this=0x55555653fdc0) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:207
#28 0x00007ffff71fad04 in synfig::Node::on_child_changed(synfig::Node const*) (this=0x55555653fdc0, x=0x555558812770) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:349
#29 0x00007ffff71fa2b6 in synfig::Node::child_changed(synfig::Node const*) (this=0x55555653fdc0, x=0x555558812770) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:213
#30 0x00007ffff71fac7d in synfig::Node::on_changed() (this=0x555558812770) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:341
#31 0x00007ffff722eb4b in synfig::ValueNode::on_changed() (this=0x555558812770) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/valuenode.cpp:149
#32 0x00007ffff71fa284 in synfig::Node::changed() (this=0x555558812770) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/node.cpp:207
#33 0x00007ffff722ec94 in synfig::ValueNode::replace(etl::handlesynfig::ValueNode) (this=0x555558929a20, x=...) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/valuenode.cpp:166
#34 0x00007ffff76258ee in synfig::ValueNode_Dynamic::operator()(synfig::Time) const (this=0x55555653fdc0, t=...) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/valuenodes/valuenode_dynamic.cpp:146
#35 0x00007ffff71a8332 in synfig::Layer::set_time(synfig::IndependentContext, synfig::Time) const (this=0x555556505000, context=..., time=...)
at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/layer.cpp:615
#36 0x00007ffff718a84a in synfig::IndependentContext::set_time(synfig::Time, bool) const (this=0x7fffffffa3c8, time=..., force=false) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/context.cpp:111
#37 0x00007ffff7167ddd in synfig::Canvas::set_time(synfig::Time) const (this=0x555557982670, t=...) at /home/rodolfo/projetos/synfig/synfig-core/src/synfig/canvas.cpp:441
#38 0x0000555555fcd9c2 in studio::Renderer_Canvas::enqueue_render_frame(etl::handlesynfig::rendering::Renderer const&, etl::handlesynfig::Canvas const&, synfig::RectInt const&, studio::Renderer_Canvas::FrameId const&) (this=0x555557c87630, renderer=..., canvas=..., window_rect=..., id=...) at /home/rodolfo/projetos/synfig/synfig-studio/src/gui/workarearenderer/renderer_canvas.cpp:458
#39 0x0000555555fcec6b in studio::Renderer_Canvas::enqueue_render() (this=0x555557c87630) at /home/rodolfo/projetos/synfig/synfig-studio/src/gui/workarearenderer/renderer_canvas.cpp:590
#40 0x0000555555fd02fa in studio::Renderer_Canvas::render_vfunc(Glib::RefPtrGdk::Window const&, Gdk::Rectangle const&) (this=0x555557c87630, drawable=..., expose_area=...)
at /home/rodolfo/projetos/synfig/synfig-studio/src/gui/workarearenderer/renderer_canvas.cpp:771
#41 0x000055555614da04 in studio::WorkArea::refresh(Cairo::RefPtrCairo::Context const&) (this=0x555557db6580) at /home/rodolfo/projetos/synfig/synfig-studio/src/gui/workarea.cpp:1922