Fix OpenGL video backend when using "render to main" on Windows.

[CrossVR]

now that the renderer doesn't create it's own window anymore it doesn't have to care about who is the parent of who and how the window should be sized, that's now handled by the GUI like it should be
but even with the previous code, what was done in OGL really should've been the responsibility of the EmuWindow

[delroth]

@dolphin-emu-bot rebuild

(I'll add you to the developers group so the buildbot autobuilds your PRs)