CFrame: Check borderless fullscreen setting before enabling exclusive fullscreen in the video config.

Fixes a bug where "Use Fullscreen" would initialize into exclusive fullscreen regardless of the borderless fullscreen setting.

Also relieves the need for the video renderer to check the borderless fullscreen setting each time.
CFrame: Don't check the video config fullscreen setting.

Checking this flag could sometimes incorrectly have the UI assume fullscreen is already off when we're still exiting.
VideoConfig: Ignore Borderless Fullscreen setting when the backend does not support exclusive fullscreen.

This was expected to be handled by VerifyValidity(), but that only verifies the validity of the INI files.