Massive refactoring of image conversion.

Massive refactoring of image conversion.

This drops all image color space and pixel format conversions from the
mlt_frame class. Instead, it adds a convert_image virtual function to
the mlt_frame class that is called within mlt_frame_get_image(). The
newly added imageconvert filter sets that virtual function and contains
the various conversion routines. The loader producer automatically
attaches this filter to the producer it creates.

Signed-off-by: Dan Dennedy <dan@dennedy.org>