Do this instead of exporting raw_pix_fmt_tags
via avpriv_get_raw_pix_fmt_tags().
For shared builds this will lead to an increase in the
combined size of the lavc and fourcc2pixfmt binaries
(because the overhead of exporting avpriv_get_raw_pix_fmt_tags()
is dwarfed by the size of the array), but given that
fourcc2pixfmt is a test tool that is not widely distributed
it does not really matter. For static builds the opposite
is true (the rest of lavc/raw.o is no longer pulled into
the test tool and the getter can be removed, too).
This patch has the additional benefit of removing
struct PixelFormatTag from the ABI.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>