The fits decoder decodes to native pixel formats; so
the fitsdec-gbrap16be fate test failed on BE despite
its name because the reference file is LE.
This patch fixes this by forcing a pixel format;
the forced pixel format is BE, causing a change
in the reference file.
The fitsdec-gbrp16be test was not affected, because
its source file (lena-rgb48.png from tne FATE suite)
is actually biendian (as if someone had multiplied
8bit content by 257...).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>