This separates the test suite from the random gtk+ using test
programs. "demos" is somewhat misleading because the programs there
are not particularly exciting (with the possible exception of
composite-test which shows off all the compositing operators).

Non-repeating gradient images would be reported as opaque. Also add
new test program to test source clipping.

Julien Cristau <jcristau@debian.org>.

Rename pixman_image_composite() to pixman_image_composite_rect()