gdk-pixbuf: update 2.40.0 -> 2.42.6

Note: jpg/png loaders are now builtin and no longer
provided as separate packages.

Drop backport missing-test-data.patch.

Drop no longer needed 0004-Do-not-run-tests-when-building.patch

Rebase 0004-Do-not-run-tests-when-building.patch

Rework 0003-target-only-Work-around-thumbnailer-cross-compile-fa.patch
so that it can be applied to both native and target builds.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>