Refactor context2d thread logic

Refactor context2d thread logic

1. Use QQuickContext2DRenderThread for Threaded rendering
2. Make FBO target works with all render strategies
3. Remove some unnessary locks, call texture methods by invoking meta calls
4. Run existing tests with all render targets and strategies (except Cooperative)

Change-Id: I0db5c91d848b86bcc1536c30d7a5804b66a817f1
Reviewed-by: Yunqiao Yin <charles.yin@nokia.com>