mask compositor: Set a check_composite method

Commit 503b6b9e2ea6 added a check_composite method to the mask compositor, but
only added it to one of the existing implementations. This commit fixes that.

In cairo-image-compositor.c, there is already a check_composite method which
just returns success for the traps compositor. This commit makes the mask
compositor use that one.

I don't want to say much about cairo-image-mask-compositor.c except that I
wondered why this file and the file above both define a non-static function
called _cairo_image_mask_compositor_get(). In my opinion, that file should just
be deleted, since it confuses e.g. ctags, but I'll let someone else clean this
up.

Fixes 493 crashes in the test suite for the test-mask target.

Signed-off-by: Uli Schlachter <psychon@znc.in>
Tested-by: Bryce Harrington <b.harrington@samsung.com>
mesh-rasterize: Fix number of iterations

Commit 44a09f462c fixed a compiler warning, but changed the result of this code.
This is because the old 'for' loop did one more iteration than the new 'while'
loop. Fix this by incrementing the loop counter once before the loop.

Fixes: mesh-pattern mesh-pattern-accuracy mesh-pattern-conical
mesh-pattern-control-points mesh-pattern-fold mesh-pattern-overlap
mesh-pattern-transformed record-mesh

Signed-off-by: Uli Schlachter <psychon@znc.in>
Tested-by: Bryce Harrington <b.harrington@samsung.com>