fate: Avoid unnecessary pixel format conversions

Most of the fate-dds-* and fate-txd-* tests already
output into the same pixel format regardless of
platform endianness, so there's no need to force
conversion to another format.

This fixes the tests fate-txd-16bpp, fate-txd-odd,
fate-dds-rgb16, fate-dds-rgb24 and fate-dds-xrgb on
big endian, where the tests seem to fail due to issues
with certain conversion codepaths in swscale.

Those conversion codepaths should of course be fixed, but
the individual decoder tests should use as little extra
conversion steps as possible.

Signed-off-by: Martin Storsjö <martin@martin.st>