vo_opengl: fix theoretical format mismatch issue

This would have been wrong for hw decoders which pass us NV12 or NV21.
The format the GL shader filter chain gets is stored in p->image_desc,
while p->image_format still contains the "real" input format (which in
case of hw decoding is an opsque hw accel format). Since no hw decoder
did this, this is really just a theoretical fix and doesn't fix any
actual bugs.