Requires GEGL git master.

Rename the 'basic_usage' test to 'basic_read' and add 'basic_write'
test. Implement GimpTileBackendTileManagerPrivate enough to make them
pass.

Note that these tests currently do not work with GIMP's "effective
tile size" optimization.

Use g_test_add_func() and g_test_add_data_func().

and no need to initialize the entire struct explicitly.

This reverts commit 23907a29.

This is to replace the old checks which were removed in
commit ff6727ae.

Keeping the checks raises red flags in static analysis as render_func
can then be NULL.

but don't fix it for item trees yet (refactoring only). Kill the
"exclusive liked" function which only existed because it was so easy
to have, but was always utterly useless. Prove me wrong and I will
revive it.

This reverts commit c0083d24. It was
decided that although this code doesn't have an effect, it needs to be
modified to be fixed.

This reverts commit 3793c1f1.

This reverts commit cea29187.