Supply user-accessible version of packing algorithm

[kenrussell]

(The text of this issue came largely from an old Khronos-internal bug.)

The current editor's draft of the WebGL specification requires that the packing algorithm defined in GLSL ES 1.0.17, Appendix A, Section 7 "Counting of Varyings and Uniforms" succeed when run against shaders and programs. See https://www.khronos.org/registry/webgl/specs/latest/1.0/#PACKING_RESTRICTIONS .

In a conference call from 29 Nov 2012 (embarrassingly old), there was a request from a working group member for a user-visible version of the packing algorithm that operates upon representations of the types and numbers of the uniforms and varyings, as well as the limits to be imposed, and which provides a boolean answer. (It wasn't a request for a JavaScript compiler for GLSL shaders.)

To avoid duplication and skew between this library and how the browser works, however, the most robust way of providing this functionality may be to compile ANGLE's shader translator with Emscripten or a forthcoming WebAssembly toolchain, and invoke it directly from ECMAScript.

This tool hasn't been shown to be strictly necessary but could still be useful; for example, shader authors could experiment with compiling their shaders given various limitations to see on how small a device they could successfully run.