webgl: Add isProgram() support.

There's one failure still, where a deleted program should still be
considered to be a program until it's unbound.  However, I recently
made it so that we unbind at delete time, and we may need to partially
back that change out.

components/script/dom/webglprogram.rs

@@ -87,6 +87,10 @@ impl WebGLProgram {
         }
     }
 
+    pub fn is_deleted(&self) -> bool {
+        self.is_deleted.get()
+    }
+
     /// glLinkProgram
     pub fn link(&self) {
         self.linked.set(false);