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
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
Indicate headless compositor presence in config.h

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
compositor: Document options for headless compositor

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
headless: Be a bit verbose in our logging

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Add a temporary script for running the headless compositor

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>