Implement parallel_invoker

[johannesugb]

Currently, there is only one invoker: class sequential_invoker which invokes invokee::initialize(), invokee::update(), invokee::render(), etc. sequentially. Vulkan, however, allows to build command buffers in parallel to decrease the CPU-time of a frame by utilizing multiple CPU cores.

Implement class parallel_invoker : public invoker_interface and invoke all of the invokee's callback methods from multiple threads in parallel.
Attention: Only the invokees that have the same execution order may be invoked in parallel (=> evaluate invokee::execution_order()!). invokees with different execution orders must be invoked sequentially in order to maintain correctness.

A parallel_invoker can be used by providing it to the call of gvk::start as a parameter:

gvk::start( gvk::parallel_invoker(), ... );

Furthermore, create a new example application which can potentially benefit from a parallel_invoker. That can be, for example, an application which animates or transforms a lot of objects on the GPU and/or needs to create a lot of command buffers. Maybe also look into the creation of secondary command buffers for this example application!

It would be a nice-to-have if the same invokees would always be invoked from the same thread::id (i.e. across subsequent frames). In the general case, this is probably infeasible => care must be taken about the way how class descriptor_cache assigns descriptor_pools to specific thread::ids. It is unclear what happens if a descriptor_pool was created from thread 1, but in a subsequent frame, is used from thread 2, for example. Think about possible problematic cases!

Definition of done:

• gvk::parallel_invoker has been implemented and can be used as a replacement to gvk::sequential_invoker wherever the latter has been in use.
• All the example applications still work when a gvk::parallel_invoker is used with them instead of a gvk::sequential_invoker.
• An additional example application has been implemented which benefits from a gvk::parallel_invoker performance-wise.
• The new class gvk::parallel_invoker and all its methods are well documented and the Contribution Guidelines have been followed.