Update glslang version (shader-slang#940)

* Update glslang

This moves to a version of glslang that is hosted with the slang project and that includes a patch for a high-priority fix that hasn't been upstreamed into the main glslang repository yet.

* Change a GLSL extension name

The glslang codebase changed the extension name required to enable certain features from `GL_KHX_shader_explicit_arithmetic_types` to `GL_EXT_shader_explicit_arithmetic_types`.

Author: Tim Foley

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "external/glslang"]
 	path = external/glslang
-	url = https://github.com/KhronosGroup/glslang.git
+	url = https://github.com/shader-slang/glslang.git
 [submodule "external/tinyobjloader"]
 	path = external/tinyobjloader
 	url = https://github.com/syoyo/tinyobjloader

external/glslang

@@ -86,8 +86,7 @@ void requireGLSLHalfExtension(ExtensionUsageTracker* tracker)
         requireGLSLExtension(tracker, "GL_EXT_shader_16bit_storage");
 
         // https://github.com/KhronosGroup/GLSL/blob/master/extensions/ext/GL_EXT_shader_explicit_arithmetic_types.txt
-        // Use GL_KHX_shader_explicit_arithmetic_types because that is what appears defined in glslang 
-        requireGLSLExtension(tracker, "GL_KHX_shader_explicit_arithmetic_types");
+        requireGLSLExtension(tracker, "GL_EXT_shader_explicit_arithmetic_types");
 
         tracker->hasHalfExtension = true;
     }

source/slang/emit.cpp

@@ -15,8 +15,8 @@ standard output = {
                               ExecutionMode 4 LocalSize 4 4 1
                               Source GLSL 450
                               SourceExtension  "GL_EXT_shader_16bit_storage"
+                              SourceExtension  "GL_EXT_shader_explicit_arithmetic_types"
                               SourceExtension  "GL_GOOGLE_cpp_style_line_directive"
-                              SourceExtension  "GL_KHX_shader_explicit_arithmetic_types"
                               Name 4  "main"
                               Name 9  "pos_0"
                               Name 13  "gl_GlobalInvocationID"