Improvements to WEBGL_draw_buffers extension test

Clarifications, tighter checks for parameters (according to the WebGL
extension spec, MAX_COLOR_ATTACHMENTS_WEBGL should be at least 4 and at
least the same as MAX_DRAW_BUFFERS_WEBGL) and a typo fix.
Small fix to WEBGL_draw_buffers extension test

A short array of attachments is allowed, there was partial code for
checking this which will work after this commit.
Clarification to draw buffers test explanation

Technically, draw buffer state does change when FB binding changes, but
the contents of the attachments (which they may share) stay the same.
This small issue caused a lot of head-scratching...
Merge pull request #617 from NVIDIA/max-draw-buffers-check

Improvements to WEBGL_draw_buffers extension test