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

Adds wl_test_record_screenshot() to weston test.  This commit also
adds a dependency on cairo to weston-test to use it for writing PNG
files.