gsk: Always use RGBA on big-endian architectures

gsk: Always use RGBA on big-endian architectures

The memorytexture test started failing on big-endian CPUs while using
"desktop" GL (not GLES) in commit b0e26873 "gsk: Use has_bgra in more
places". The subsequent commit f5812808 "gsk: Restore bigendian support"
was meant to fix this, but it seems it did not.

Always taking the RGBA code-path on big-endian might not be optimal,
but it seems to result in the test passing again.

Signed-off-by: Simon McVittie <smcv@debian.org>
Bug: https://gitlab.gnome.org/GNOME/gtk/-/issues/6260
Bug-Debian: https://bugs.debian.org/1057782
Forwarded: https://gitlab.gnome.org/GNOME/gtk/-/merge_requests/6653

Gbp-Pq: Name gsk-Always-use-RGBA-on-big-endian-architectures.patch