Clarifications and small fixes to GLSL references in WebGL 2

Refer to GLSL ES 3.00 spec which more clearly states GLSL ES 1.00 draw
buffers restrictions in GLES 3.0, and fix GLSL 3.0 -> GLSL ES 3.00.
Merge pull request #678 from NVIDIA/webgl2-glsl-es-fix

Clarifications and small fixes to GLSL references in WebGL 2
Log more information to the console.

This is principally needed to debug issues on the Chromium project's automated test machines.
Added an error catch to all instances of canvas.toDataUrl, to properly report intermittent failures seen in Chrome.
Merge pull request #679 from kenrussell/log-debug-to-console

Log more information to the console.
Addressed freedback from Ken
Merge pull request #680 from toji/to_data_url_error

Added an error catch to all instances of canvas.toDataUrl
Forgot to add "FAIL" to failing messages logged to console.
Merge pull request #681 from kenrussell/fail-message

Forgot to add "FAIL" to failing messages logged to console.
Added more debug logging to the premultiply alpha test.
Merge pull request #682 from toji/premultiply-alpha-logging

Added more debug logging to the premultiply alpha test.
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
Reduce logging back to previous levels for most tests.

The dramatic increase to the amount of logging these tests do is killing the Chromium project's tryserver master. Reduce the pass, debug, and description logging back to their previous levels by default. Keep them all increased for the tests being actively debugged in http://crbug.com/393331 and http://crbug.com/407976 .
Added verbose logging back to texture-upload-size.html.
Merge pull request #687 from kenrussell/reduce-logging

Reduce logging back to previous levels for most tests.