Verify if component values are set correct

GLSL constructors : Added code to verify if component values are set
correct by constructor expression.
Use union types instead of overloading for buffer[Sub]Data and tex[Sub]Image2D.

This fixes https://www.khronos.org/bugzilla/show_bug.cgi?id=1172 .
Added text to all new interfaces except WebGLSync, which I'm not terribly familiar with
Made the latest spec landing page slightly more useful
Added desriptions of VertexArray functions
Added conformance test for EXT_blend_minmax
Added BufferDataSource typedef on suggestion from Brandon Jones.
Merge pull request #562 from kenrussell/union-types

Use union types instead of overloading for buffer[Sub]Data and tex[Sub]I...
Merge pull request #567 from toji/blend_minmax_conformance

Added conformance test for EXT_blend_minmax
Shorter comparison fp component validity check

More compact comparison for floating point component validity check by
using abs() instead of separate positive and negative comparison
encapsulates -> encapsulate
Merge pull request #565 from toji/webgl2-spec-interfaces

Added text to all new WebGL2 interfaces except WebGLSync
Fixed title
Merge pull request #566 from toji/spec-landing

Made the latest spec landing page slightly more useful
Merge pull request #535 from TiborsHub/master

Verify if component values are set correct