Release v6.2.1

Therion 6.2.1 (2024-03-20):

therion:

• Polish translation updated [#553]
• many C++/CMake/Makefile improvements and fixes by Matěj Plch
  [#535, #536, #537, #539, #541, #541, #542, #552]
• updated poly2tri library [#538]
• bugs fixed:
  • length can also be equal to depth change [#556]
  • std::sort() usage causing segmentation fault in some cases

xtherion:

• new setting $xth(gui,init_app_normalized) to allow disabling of window
  geometry adjustments when xtherion starts [#557]
• xtherion line artifact fixed [#554]

infrastructure:

• initial implementation of a devcontainer [#532]
• CI environments: fixes, macOS 13 added [#546, #551]