Make ref tests check the pixels for differences and emit a difference file.
The compositor needs to not process a Paint message after it has started
shutdown or it will crash creating a native layer. This happened during
reftests using -o to emit a PNG of the backbuffer and then exiting early.
auto merge of #1544 : larsbergstrom/servo/improve-reftesting, r=metajack

r? @metajack