compositor-drm: Support overlays with transformed buffers

compositor-drm: Support overlays with transformed buffers

Make overlays work when the client uses a buffer with the same
transformation as the output.

In order to calculate the destination rectangle, the same logic in
weston_surface_to_buffer_float() is needed, but with the output
dimensions instead. For that reason, this patch generalizes this
function into weston_transformed_{coord,rect} and moves it to util.c.
The surface functions are then implemented using those.