hwc: Handle buffer sync and skip validate for Layer Stitch.

-- Input buffers can be released as and when they get copied
   to stitch target.
-- Deprecate sync_merge API usage.
-- Init GLContext for LayerStitch post IDisplayConfig start.

CRs-Fixed: 2589145
Change-Id: I913470b7197007df9d8dae2a4b232d6283c3a4cb
18 files changed