vout: apple: add option to force fallback to libvlc legacy displays

The recent addition of the new display to support natively decoded CVSampleBufferRef frames provides straightforward Metal rendering but may still suffer from unsupported features or unstable code.

Being able to fallback to any legacy OpenGL based display might be helpful to get around unexpected issues with the new display.

In libvlc, vout displays are opened by order of priority and can't be set using the --vout option.

Libvlc users had no way to use an alternate vout display if the samplebufferdisplay doesn't work as expected.

This change is introducing a new option for libvlc users to force a fallback to legacy OpenGL based displays.