avimux: add RGB16 support

Submitted by Julien Isorce @cap

Link to original bug (#723358)

Description

Created attachment 267721
avimux: add RGB16 support

• actual result: avimux does not support RGB16

• random riff kind of spec I found:
  http://netghost.narod.ru/gff/vendspec/micriff/ms_riff.txt I can read "RGB565"

• with the attached patch I generated :
  gst-launch-1.0 videotestsrc num-buffers=200 ! "video/x-raw, format=RGB16" ! avimux ! filesink location=rgb16.avi

Then vlc can play it

Patch 267721, "avimux: add RGB16 support":
0002-avimux-add-RGB16-support.patch