backends: refactor transform string parsing

Most of the backends do their own parsing of transform strings, so let's
put that all in the same place (compositor.c/h)
compositor-headless: allow rendering with pixman

Allow the headless backend to render into an off screen buffer with pixman.
compositor-headless: add support for transforms set on command line

We now allow --transform with the headless compositor, however it still
doesn't parse anything out of weston.ini
tests: Allow tests to use customized command line parameters

Tests will now return the extra command line parameters they need
when run with --params
tests: add screenshot recording to weston-test
tests: add test of headless compositor screenshot mechanism