Separate GLSLANG_WEB (min-size build) and Emscripten options

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.

glslang/OSDependent/Web/glslang.pre.js

@@ -0,0 +1,45 @@
+Module['compileGLSLZeroCopy'] = function(glsl, shader_stage, gen_debug) {
+    gen_debug = !!gen_debug;
+
+    var shader_stage_int;
+    if (shader_stage === 'vertex') {
+        shader_stage_int = 0;
+    } else if (shader_stage === 'fragment') {
+        shader_stage_int = 4;
+    } else if (shader_stage === 'compute') {
+        shader_stage_int = 5;
+    } else {
+        throw new Error("shader_stage must be 'vertex', 'fragment', or 'compute'");
+    }
+
+    var p_output = Module['_malloc'](4);
+    var p_output_len = Module['_malloc'](4);
+    var id = ccall('convert_glsl_to_spirv',
+        'number',
+        ['string', 'number', 'boolean', 'number', 'number'],
+        [glsl, shader_stage_int, gen_debug, p_output, p_output_len]);
+    var output = getValue(p_output, 'i32');
+    var output_len = getValue(p_output_len, 'i32');
+    Module['_free'](p_output);
+    Module['_free'](p_output_len);
+
+    if (id === 0) {
+        throw new Error('GLSL compilation failed');
+    }
+
+    var ret = {};
+    var outputIndexU32 = output / 4;
+    ret.data = Module['HEAPU32'].subarray(outputIndexU32, outputIndexU32 + output_len);
+    ret.free = function() {
+        Module['_destroy_output_buffer'](id);
+    };
+
+    return ret;
+};
+
+Module['compileGLSL'] = function(glsl, shader_stage, gen_debug) {
+    var compiled = Module['compileGLSLZeroCopy'](glsl, shader_stage, gen_debug);
+    var ret = compiled.data.slice()
+    compiled.free();
+    return ret;
+};