For reverse PRIME heads, scaling was broken as the compositing of the
shared pixmaps wasn't properly configured to take into account scaling
(and other transformations). For example, by scaling a reverse PRIME
head by 2x2 the user would only see the top-left quadrant of the 2x2
screenspace instead of the full screenspace scaled down to the size of
the shared pixmap.

This change brings the RandR transformation available for tracking and
updating of dirty regions, and subsequently the CompositeRotate()
function which renders the transformation onto the shared pixmap. For
transforms with simple translations only a separate execution path is
tracked so that direct copying can be used.

These changes should fix scaling, and to some extent, arbitrary
transformations, for all heads that are reverse PRIME sinks.

Note that they only clean up certain parts of the viewport
transformation and composition pipeline. A more thorough overhaul of
how all the offsets and transformations are managed would be in order
to make all combinations of randr screen configurations work
seamlessly. I tried to be careful not to break existing functionality
but there are things that were broken in the first place and remain to
be so.

As a quick test, basic scaling like this should now work properly with
reverse PRIME outputs:

  xrandr --output <X> --scale 0.5x0.5
  xrandr --output <X> --scale 2x2

Arbitrary transforms still exhibit various quirks and artifacts but
something like a basic free rotation should now behave a bit better
than before with reverse PRIME outputs:

  xrandr --output <X> --transform 0.707,-0.707,0,0.707,0.707,0,0,0,1