modesetting: apply transforms in PRIME shared pixmap updates

For reverse PRIME heads, with 'modesetting' as the source, 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 in the modesetting driver, and subsequently
the CompositeRotate() function which renders the transformed screen to
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 driven
from 'modesetting' source.

Note that these changes 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

With this change, this command yields a screen that is at least now
rotated yet still wrongly clipped as it was also before this change.
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