Consider exposing EXT_pvrtc_sRGB to WebGL

Consider exposing [EXT_pvrtc_sRGB](https://www.khronos.org/registry/OpenGL/extensions/EXT/EXT_pvrtc_sRGB.txt) to WebGL.

Discussion here:
https://www.khronos.org/webgl/public-mailing-list/public_webgl/1707/msg00027.php
> iOS 7.0 devices, including PowerVR SGX 543 and 554 supports sRGB PVRTC throught EXT_pvrtc_sRGB extensions.

Read the linked EXT_pvrtc_sRGB specification and the Khronos WebGL public mailing-list discussion first. The issue does not name implementation files or tests; done would require a decided scope for exposing the extension and corresponding project changes.

Written by the indexing model from the issue text.