Add draft extension proposal for WEBGL_subscribe_uniform.

Draft proposal for an extension that decreases perceived input latency. I will send out an accompanying email to describe motivation, background and solicit further feedback.
WebGLValueBuffer needs to be defined in the IDL.

Also, please describe in comments why this new type and all of these new APIs for managing Valuebuffers are being added. If UBOs will support this use case in WebGL 2.0, please mention that, and describe how the API would be changed or simplified for WebGL 2.0.