.NET 5 build cannot run on OSX 10.15.7

[cousinbenson]

Issue Summary

OSX Catalina 10.15.7 cannot run net5 builds due to the following error:
-[NSOpenGLContext setView:] must be called from the main thread.

Reproduction

On OSX 10.15.7:

• Install .NET 5
• Run make
• Run ./launch-game.sh Game.Mod=ra
• Observe crash

Expected behavior

Game should run.

Screenshots / Screen recordings / Replays

[Mailaender]

Same problem here https://pastebin.com/AXeYBLvd using a .dmg build.

[pchote]

I did some digging here and found that this appears to be related to a change in the macOS 10.15 platform SDK that turned a warning into a runtime error. mono is likely built against the old SDK and dotnet against the new one, explaining why the compatibility builds continue to work.

libsdl-org/SDL@309d613 was shipped in SDL 2.0.14 (we curently use 2.0.12) and should fix the issue. I attempted to test the latest SDL (now 2.0.16) in a macOS 10.15.7 virtual machine, but it seems that there were other changes that aren't compatible with the emulated software GL implementation in the virtual machine - I expect this won't be a problem with real hardware.