lavd/x11grab: Support AV_PIX_FMT_PAL8.

Fixes ticket #3068.
Merge remote-tracking branch 'cehoyos/master'

* cehoyos/master:
  lavd/x11grab: Support AV_PIX_FMT_PAL8.

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