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