Add a proposal for a WEBGL_debug_shader_precision extension

This proposal is based on implementing a prototype of the extension's
functionality for mediump operations in ANGLE. The emulation has been
proven to be implementable and useful, and most shaders it has been tried
with have been able to compile and run at interactive rates on a
mid-range GPU.
Merge pull request #778 from NVIDIA/debug-shader-precision-extension

Add a proposal for a WEBGL_debug_shader_precision extension