Add missing explanations for sync related functions

There's still an open question about whether waitSync for waiting on the
GL server is at all useful for WebGL, but document it alongside the other
sync functions.
Merge pull request #683 from NVIDIA/webgl2-sync-spec

Add missing explanations for sync related functions
Add a test case to ensure drawBuffers([gl.NONE]) does not prevent internal drawbuffer clearing after compositing.

The test case is provided by jmuizelaar@mozilla.com.
Merge pull request #685 from zhenyao/mrt

Add a test case to ensure drawBuffers([gl.NONE]) does not prevent intern...
Update wtu.checkCanvas() to print out the actual color

This definitely helps us to better understand a failure.
Merge pull request #686 from zhenyao/pixel

Update wtu.checkCanvas() to print out the actual color