Auto push constant blocks (#2764)

* add ability to upgrade uniform block to push constants assuming it fits within the size limit imposed by the caller
* allow selecting the packing for the auto push constants
* check the size using the potential layout packing of the push constants

glslang/Include/Types.h

@@ -741,6 +741,16 @@ public:
         }
     }
 
+    bool isUniform() const
+    {
+        switch (storage) {
+        case EvqUniform:
+            return true;
+        default:
+            return false;
+        }
+    }
+
     bool isIo() const
     {
         switch (storage) {