Clean up shader-varying-packing-restrictions.html

Fix typos, clarify a few comments and remove dead code.
Modify WEBGL_draw_buffers extension spec.
Per kbr's review
Merge pull request #353 from zhenyao/mrt

Modify WEBGL_draw_buffers extension spec.
Backport shader-varying-packing-restrictions.html cleanup to 1.0.2

This fix does not change test behavior.
Merge pull request #351 from NVIDIA/varying-cleanup

Clean up shader-varying-packing-restrictions.html