Add a few vec/mat constructor corner cases

To make sure ANGLE SH_SCALARIZE_VEC_AND_MAT_CONSTRUCTOR_ARGS workaround
works as expected.
Update copyright notice year.
Relax numeric comparison a bit to be more tolerant of precision.
Merge pull request #625 from zhenyao/construct

Add a few vec/mat constructor corner cases
Add information on range checking and primitive restart in WebGL 2

Includes filling out some differences between WebGL 1 and WebGL 2.
Typo fixes
WebGL2: IsEnabled docs and formatting for "Setting and getting state"

RASTERIZER_DISCARD is missing from the GLES3 man pages, but is in the
spec, so add that to the list of queriable parameters. Add spec link for
getParameter.
Merge pull request #627 from NVIDIA/primitive-restart-range-check

Add information on range checking and primitive restart in WebGL 2