D3D: Check borderless fullscreen setting before initializing with exclusive fullscreen.

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