OSmesa is deprecated and has been removed from latest mesa releases, this replaces it with pbuffer rendering and flushing of the surface onto the bitmap at some specific sync points. The tests show that this is roughly how Windows seem to behave anyway, instead of rendering directly to the memory as OSmesa does.
-- v2: win32u: Remove now unnecessary context and pbuffer funcs. win32u: Drop now unnecessary OSMesa dependency. win32u: Use a pbuffer to implement GL on memory DCs. opengl32: Expose every pixel format on memory DCs. winex11: Check XVisualInfo vs GLXFBConfig depth to avoid BadMatch. winex11: Flag offscreen formats as bitmap compatible if possible. winemac: Use opengl_funcs for glFlush. winex11: Use opengl_funcs for glFlush / glFinish. opengl32: Move glFlush / glFinish hooking from win32u. win32u: Avoid closing NULL egl_handle on dlopen failure.