Bug 24111119

This is a trivial implementation which does not actually optimize
compositing using the damage rects. That can be implemented later.

Change-Id: Ibf511ef166a33b1d2f1826bfc5618765062fe2a2
Reviewed-on: https://swiftshader-review.googlesource.com/4014Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>