* Introduces builtin variables gl_SecondaryFragColorEXT and gl_SecondaryFragDataEXT
* Introduces builtin constant gl_MaxDualSourceDrawBuffersEXT
* enables support for layout qualifier "index" in es profile
This should enable glslang to be used as a dependency of other projects
that target WebAssembly, instead of assuming "EMSCRIPTEN" => "Want
glslang.js".
Allows building non-min-size builds with Emscripten.
Adds ENABLE_GLSLANG_WEB_DEVEL.
Moves the glslang.js files to OSDependent/Web.
Small cleanups and docs update.