txd: return meaningful error codes.
cyuv: return meaningful error codes.
eatgv: return meaningful error codes.
eatgv: cosmetics, reformat
eatgq: return meaningful error codes.
eatgq: cosmetics, reformat.
dxa: return meaningful error codes.
pngdec: cosmetics, reformat.

Also remove a commented out line.
iff: drop ff_ prefix from a static function.
mimic: cosmetics, reformat
mimic: return meaningful error codes.
rawdec: cosmetics, reformat
mjpegdec: fix indentation
mpegvideo_enc: do not modify the input frame.
mpegvideo_enc: fix indentation in load_input_picture()