Bug#1094844: gtk4: 4.17.3 fails to build on armel & armhf: memorytexture & scaling test failures

[Jeremy Bícha]

Control: severity -1 important
Control: unblock 1094845 by -1
Control: user debian-arm at lists.debian.org
Control: usertags -1 armel

This build failure was a result of a packaging change where we now
install mesa-vulkan-drivers which enabled running additional tests.
Since there is no regression here, in our next upload, we simply won't
install mesa-vulkan-drivers for use by those build tests. Therefore,
we can lower the severity and this is no longer blocking uploading
gtk4 4.17/4.18 to Unstable.

There is likely an underlying bug in Gdk.MemoryTexture that needs to
be looked into though.

Thank you,
Jeremy Bícha