drm_hwcomposer: support virtual displays

Virtual display support is mandatory for HWC version 1.4.

As a minimal implementation, we use frambuffer target and let SF take care of
GLES composition. We still need to handle the acquire and release fences for
both the virtual display and all layers in a separate worker thread.

TEST=screen casting to chromecast

Change-Id: Idb962f4b5bb852c9ec9ebcaa1679a653c01737fb
4 files changed