Clarify EXT_texture_norm16

[lexaknyazev]

• The WebGL extension spec lists support for UNSIGNED_SHORT textures as FBO attachments as a WebGL-specific feature although this seems to be already included in the GLES extension.
• The WebGL extension spec lists only texImage2D and texSubImage2D as the extended entrypoints while the GLES extension explicitly includes also 3D, immutable, and renderbuffer functions. To avoid any ambiguity, the WebGL extension should either only link to the GLES extension or have the same explicit list. On top of that, the missing functions are not covered by the CTS.

[kdashg]

I think the webgl-specific thing is that we would accept e.g. RGBA/RGBA/UNSIGNED_SHORT. I believe a strict reading of the ES extension only supports e.g. RGBA16F/RGBA/UNSIGNED_SHORT. This could be rephrased better.

And yeah, this should probably say "texImage and texSubImage".