lavd/opengl_enc: add gray8/16 formats

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
lavd/opengl_enc: implement uncoded frame callback

Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>
Merge remote-tracking branch 'lukaszmluki/master'

* lukaszmluki/master:
  lavd/opengl_enc: implement uncoded frame callback
  lavd/opengl_enc: add gray8/16 formats

Merged-by: Michael Niedermayer <michaelni@gmx.at>