Path: fix X/Y mixup on rects

All rectangles added to path were turned into squares. This was spotted
by WebKit canvas tests.
Add support for pixel dumps

* The offscreen bitmap is converted to PNG with the translation kit and
dumped to standard output by DumpRenderTree.
* The ImageDiff tool is used to compare two PNG files given on the
command line. It computes a difference percentage, and if that's too
high, also generate a difference map to help spotting the errors.

There is a memory leak in ImageDiff, but I can't find it. Help welcome.