builtin atomics need element() operation

Author: Carlos Scheidegger

src/shade/fragCoord.js

@@ -7,8 +7,11 @@ Shade.fragCoord = function() {
         evaluate: function() {
             throw new Error("evaluate undefined for fragCoord");
         },
+        element: function(i) {
+            return this.at(i);
+        },
         compile: function(ctx) {
         },
-        json_key: function() { return "fragCoord"; }
+        _json_key: function() { return 'fragCoord'; }
     });
 };

src/shade/pointCoord.js

@@ -9,6 +9,9 @@ Shade.pointCoord = function() {
         evaluate: function() {
             throw new Error("evaluate undefined for pointCoord");
         },
+        element: function(i) {
+            return this.at(i);
+        },
         _json_key: function() { return 'pointCoord'; }
     });
 };