Add czm_backFacing

Author: lilleyse

Source/Scene/processModelMaterialsCommon.js

@@ -643,7 +643,7 @@ function generateTechnique(
     fragmentShader += "  vec3 normal = normalize(v_normal);\n";
     if (khrMaterialsCommon.doubleSided) {
       // !gl_FrontFacing doesn't work as expected on Mac/Intel so use the more verbose form instead. See https://github.com/CesiumGS/cesium/pull/8494.
-      fragmentShader += "  if (gl_FrontFacing == false)\n";
+      fragmentShader += "  if (czm_backFacing())\n";
       fragmentShader += "  {\n";
       fragmentShader += "    normal = -normal;\n";
       fragmentShader += "  }\n";

Source/Scene/processPbrMaterials.js

@@ -768,7 +768,7 @@ function generateTechnique(
     }
     if (material.doubleSided) {
       // !gl_FrontFacing doesn't work as expected on Mac/Intel so use the more verbose form instead. See https://github.com/CesiumGS/cesium/pull/8494.
-      fragmentShader += "    if (gl_FrontFacing == false)\n";
+      fragmentShader += "    if (czm_backFacing())\n";
       fragmentShader += "    {\n";
       fragmentShader += "        n = -n;\n";
       fragmentShader += "    }\n";

Source/Shaders/Builtin/Functions/backFacing.glsl

@@ -0,0 +1,13 @@
+/**
+ * Determines if the fragment is back facing
+ *
+ * @name czm_backFacing
+ * @glslFunction 
+ * 
+ * @returns {bool} <code>true</code> if the fragment is back facing; otherwise, <code>false</code>.
+ */
+bool czm_backFacing()
+{
+    // !gl_FrontFacing doesn't work as expected on Mac/Intel so use the more verbose form instead. See https://github.com/CesiumGS/cesium/pull/8494.
+    return gl_FrontFacing == false;
+}

Source/Shaders/GlobeFS.glsl

@@ -453,8 +453,7 @@ void main()
 #endif
 
 #ifdef UNDERGROUND_COLOR
-    // !gl_FrontFacing doesn't work as expected on Mac/Intel so use the more verbose form instead. See https://github.com/CesiumGS/cesium/pull/8494.
-    if (gl_FrontFacing == false)
+    if (czm_backFacing())
     {
         float distanceFromEllipsoid = max(czm_cameraHeight, 0.0);
         float distance = max(v_distance - distanceFromEllipsoid, 0.0);