Fill in the spec for most texture functions in WebGL 2

Adds TEXTURE_IMMUTABLE_* enums to GetTexParameter which were previously
missing, and adds a note about TexImage3D being removed. Some parts like
format restrictions still need some work.
Added missing semicolon

As pointed out in #694.
Merge pull request #693 from NVIDIA/webgl2-texture-spec

Fill in the spec for most texture functions in WebGL 2
Fix minor issues in float texture extension specs

WEBGL_color_buffer_float had a wrong name for the interface, increasing
confusion between it and EXT_color_buffer_float. Also OES_texture_float
and OES_texture_half_float extensions had broken links.
OES_texture_half_float also incorrectly used OES prefix for
EXT_color_buffer_half_float.
Merge pull request #695 from NVIDIA/float-extension-spec-fixes

Fix minor issues in float texture extension specs