Add support for NV12 to evr_render(), as otherwise no video is rendered
at all in games like Age of Empires II DE.

In the process, the evr_copy_sample_buffer() function has been
refactored to move the MFCopyImage() call into format-specific code (to
better handle multi-planar formats cleanly).
Signed-off-by: David Gow <david@ingeniumdigital.com>