Add WEBGL_security_sensitive_resources extension proposal.

This is an extension proposal to allow cross-origin resources for texture upload. It defines some GLSL restrictions to prevent timing attacks, and it defines some WebGL APIs to manage security sensitive resources.

This extension is related to this discussion on public-WebGL:
https://www.khronos.org/webgl/public-mailing-list/archives/1211/msg00058.html