Create new threading-aware canvas class

[sbusch42]

Lets refactor the canvas and mainloop here. It should be clearly designed and documented what methods and signals are to be used from what thread (UI thread or render thread) to make the new canvas class work on multithreaded viewers such as QtQuick.

[sbusch42]

Please test this on all platforms and give feedback to the new rendering and mainloop implementations

[sbusch42]

Works on macOS

[sbusch42]

Works on Windows

[sbusch42]

Last call for feedback on this PR :)

[scheibel]

From what I can see, the new Canvas handles virtual time updates internally. However, using the video exporter (e.g., ffmpeg video export), the virtual time is handled by the exporter. I suggest we re-add an interface to handle virtual time externally.

[Tobias1595]

Right after startup the following error appears exactly 6 times on cerr:
#fatal: error: 0x500, high severity (API)
GL_INVALID_ENUM error generated. <pname> requires feature(s) disabled in the current profile.
After some searching, I found that these errors are produced by GLContextUtils::retrieveFormat().

It seems that GL_RED_BITS through GL_STENCIL_BITS are not part of the OpenGL 4.5 specification anymore (see lists in 4.5 glGet reference and 2.1 glGet reference)

Yet I haven't noticed any negative impact whatsoever though, but I also don't know, where these values are required at the moment. Neither do I know, why the very same code on the master branch runs without generating the error messages.

[sbusch42]

Thanks for reporting that. Please make a separate issue, we should investigate it further.