ext/ffmpeg/gstffmpegcodecmap.*: Change some function names to reflect that they don't really _return_ something, but ...

Original commit message from CVS:
* ext/ffmpeg/gstffmpegcodecmap.c: (gst_ffmpeg_get_palette),
(gst_ffmpeg_set_palette), (gst_ffmpeg_codecid_to_caps),
(gst_ffmpeg_pixfmt_to_caps), (gst_ffmpeg_caps_to_smpfmt),
(gst_ffmpeg_caps_to_pixfmt), (gst_ffmpeg_caps_with_codectype),
(gst_ffmpeg_caps_with_codecid), (gst_ffmpeg_caps_to_codecid):
* ext/ffmpeg/gstffmpegcodecmap.h:
Change some function names to reflect that they don't really
_return_ something, but rather _use_ something to fill a
AVCodecContext. s/to/with/. Restructure the extradata handling,
it's now not picking up the type from the caps but rather
using the type as provided in the function. This is a lot
cleaner. Implement MS RLE palette pickup.
* ext/ffmpeg/gstffmpegcolorspace.c: (gst_ffmpegcsp_pad_link):
* ext/ffmpeg/gstffmpegenc.c: (gst_ffmpegenc_connect):
Sync with the above function name changes.
* ext/ffmpeg/gstffmpegdec.c: (gst_ffmpegdec_close),
(gst_ffmpegdec_open), (gst_ffmpegdec_connect),
(gst_ffmpegdec_chain), (gst_ffmpegdec_change_state):
Add some hacks to convert palette-based raw image formats to
RGBA32. Ugly, but I don't know how else to handle palette-based
RGB, since img_convert() (and thus ffcolorspace) doesn't accept
a palette as argument.