gl-renderer: Call glViewport after the context is made current
Use pixel coordinates for weston_output.matrix

Previously, weston_output.matrix was in GL coordinates and therefore only
really useful for the GL backend.
pixman-renderer: Simplify the buffer-to-output matrix computation

Now that we have a buffer-to-surface matrix and the global-to-output matrix
is usable, we can remove a large chunk of confusing code from the pixman
renderer.  Hopefully, having this stuff in weston core will keep the pixman
renderer from gettin broken quite as often.