hwc: Avoid MDP comp for RGB downscaled layer

- In targets with fewer pipes, composition switch can happen
  continuously for a layer based on whether it is updating or not.
  If that updating layer requires downscaling, because of the
  difference in the downscale filters between MDP and GPU,
  the output of MDP and GPU will differ. This difference could be
  perceived as flicker. To avoid this flicker, mark RGB downscaled layer
  with downscale more than threshold to GPU always.
- property persist.hwc.downscale_threshold defines the threshold value
  for downscale beyond which the layer will be marked for GPU composition.

Change-Id: Ifd26d7eb1eff0096b0391a0552d0fd97386c1a19
4 files changed