Mime the ETC1 extension to handle the formats exposed by ES3 and GL4.3.
Add both ETC2 and EAC compressed texture formats.
Merge pull request #454 from jdashg/prop-ext-es3

Propose WEBGL_compressed_texture_es3 for compressed texture formats core to GLES3
In Khronos F2F meeting of WebGL WG, rewrote security considerations section as issues, and removed text forbidding exposing these extensions to arbitrary web pages.
Merge pull request #455 from kenrussell/security-considerations

In Khronos F2F meeting of WebGL WG, rewrote security considerations sect...