webgl: Detach RBs and textures from the bound FBO on deletion.

This is part of general GL behavior: when an object is deleted, look
through the currently bound objects and detach the deleted object from
them.  Detaching an object from an FBO causes it to need to be
re-checked for its status.

components/script/dom/webglframebuffer.rs

@@ -231,6 +231,7 @@ impl WebGLFramebuffer {
 
             _ => {
                 *binding.borrow_mut() = None;
+                self.update_status();
                 None
             }
         };
@@ -245,6 +246,49 @@ impl WebGLFramebuffer {
         Ok(())
     }
 
+    pub fn detach_renderbuffer(&self, rb: &WebGLRenderbuffer) {
+        let attachments = [&self.color,
+                           &self.depth,
+                           &self.stencil,
+                           &self.depthstencil];
+
+        for attachment in &attachments {
+            let matched = {
+                match *attachment.borrow() {
+                    Some(WebGLFramebufferAttachment::Renderbuffer(ref att_rb))
+                        if rb.id() == att_rb.id() => true,
+                    _ => false,
+                }
+            };
+
+            if matched {
+                *attachment.borrow_mut() = None;
+                self.update_status();
+            }
+        }
+    }
+
+    pub fn detach_texture(&self, texture: &WebGLTexture) {
+        let attachments = [&self.color,
+                           &self.depth,
+                           &self.stencil,
+                           &self.depthstencil];
+
+        for attachment in &attachments {
+            let matched = {
+                match *attachment.borrow() {
+                    Some(WebGLFramebufferAttachment::Texture(ref att_texture))
+                        if texture.id() == att_texture.id() => true,
+                    _ => false,
+                }
+            };
+
+            if matched {
+                *attachment.borrow_mut() = None;
+            }
+        }
+    }
+
     pub fn target(&self) -> Option<u32> {
         self.target.get()
     }