Bug#1118780: gtk4: Mipmap tests fail 16/32-bit pixel format conversions due to memory alignment issues

[Gregor Riepl]

> I have tested your patch now and it seems it just reduced the number of failures by one.

There's already a workaround for the "Missing JIT" error, but it seems to be broken on sparc...

https://salsa.debian.org/gnome-team/gtk4/-/blob/debian/latest/debian/rules?ref_type=heads#L279-294