webgl: Add an interface for getting the size of renderbuffers.

We need this for framebuffer status checking.

components/script/dom/webglrenderbuffer.rs

@@ -20,6 +20,7 @@ pub struct WebGLRenderbuffer {
     id: WebGLRenderbufferId,
     ever_bound: Cell<bool>,
     is_deleted: Cell<bool>,
+    size: Cell<Option<(i32, i32)>>,
     internal_format: Cell<Option<u32>>,
     #[ignore_heap_size_of = "Defined in ipc-channel"]
     renderer: IpcSender<CanvasMsg>,
@@ -36,6 +37,7 @@ impl WebGLRenderbuffer {
             is_deleted: Cell::new(false),
             renderer: renderer,
             internal_format: Cell::new(None),
+            size: Cell::new(None),
         }
     }
 
@@ -64,6 +66,10 @@ impl WebGLRenderbuffer {
         self.id
     }
 
+    pub fn size(&self) -> Option<(i32, i32)> {
+        self.size.get()
+    }
+
     pub fn bind(&self, target: u32) {
         self.ever_bound.set(true);
         let msg = CanvasMsg::WebGL(WebGLCommand::BindRenderbuffer(target, Some(self.id)));
@@ -106,6 +112,8 @@ impl WebGLRenderbuffer {
                                                                      internal_format, width, height));
         self.renderer.send(msg).unwrap();
 
+        self.size.set(Some((width, height)));
+
         Ok(())
     }
 }