SurfaceFlinger: SCALING_MODE_FREEZE cropping support

This change adss support for applying a buffer crop to layers with the
SCALING_MODE_FREEZE scaling mode.  These layers do not scale the image, but
rather treat all pixels outside the crop rectangle as fully transparent.

Change-Id: I762518e56a37aef7747f9b581df2f2589b232c49
Bug: 6299171