Inconsistent FPS stuttering when compiling against 10.14 (+?) SDK

[wraitii]

Weird bug, this. As confirmed by myself and langbart, MacOs experiences FPS stuttering without frame rate limiting in some situations.

See:

• ​https://discourse.libsdl.org/t/osx-opengl-frame-rate-issues-that-arent-frame-rate-issues/26084/12
• ​https://discourse.libsdl.org/t/sdl2-macos-mojave-opengl-frame-drops/25267
• ​https://discourse.libsdl.org/t/sdl2-no-renderer-vsync-on-macos-mojave/25169/20

From what I can tell, this doesn't happen:

• when compiled against the 10.12 SDK (as does the CI)
• when enabling High DPI in the SDL2 window
• when limiting the frame rate below some magic number, but that's less consistent.

Sync doesn't really seem to affect it.

I have no clue what's causing this, and it ought naturally disappear when we shift to High DPI, and it doesn't affect bundles because of the SDK difference, so I'll leave this open to remind ourselves.

related ticket

• #6593 (closed as duplicate, contains a lot of details)