drm_hwcomposer: add properties to disable overlays or to use HWC_FRAMEBUFFER

To use SurfaceFlinger's GL compositor, use the setprop command:
$ adb shell setprop hwc.drm.use_framebuffer_target 1

A side effect of using the above property is that drm_hwcomposer will only be
compositing one primary plane.

To disable overlays (to force the GL compositor to be used more often):
$ adb shell setprop hwc.drm.use_overlay_planes 0

The default behavior is the opposite of the above properties.

Change-Id: I86afb533c10a8caa15931e8c292f52bb94e5f6c5
2 files changed