webgl: Refactor WebGLProgram::link

This makes it more correct, since we don't blindly send the Link
command. It's not observable though.

components/script/dom/webglprogram.rs

@@ -23,6 +23,7 @@ pub struct WebGLProgram {
     webgl_object: WebGLObject,
     id: u32,
     is_deleted: Cell<bool>,
+    linked: Cell<bool>,
     fragment_shader: MutNullableHeap<JS<WebGLShader>>,
     vertex_shader: MutNullableHeap<JS<WebGLShader>>,
     #[ignore_heap_size_of = "Defined in ipc-channel"]
@@ -35,6 +36,7 @@ impl WebGLProgram {
             webgl_object: WebGLObject::new_inherited(),
             id: id,
             is_deleted: Cell::new(false),
+            linked: Cell::new(false),
             fragment_shader: Default::default(),
             vertex_shader: Default::default(),
             renderer: renderer,
@@ -71,19 +73,27 @@ impl WebGLProgram {
 
     /// glLinkProgram
     pub fn link(&self) {
-        self.renderer.send(CanvasMsg::WebGL(WebGLCommand::LinkProgram(self.id))).unwrap();
-    }
+        self.linked.set(false);
 
-    /// glUseProgram
-    pub fn use_program(&self) -> WebGLResult<()> {
         match self.fragment_shader.get() {
             Some(ref shader) if shader.successfully_compiled() => {},
-            _ => return Err(WebGLError::InvalidOperation),
+            _ => return,
         }
 
         match self.vertex_shader.get() {
             Some(ref shader) if shader.successfully_compiled() => {},
-            _ => return Err(WebGLError::InvalidOperation),
+            _ => return,
+        }
+
+        self.linked.set(true);
+
+        self.renderer.send(CanvasMsg::WebGL(WebGLCommand::LinkProgram(self.id))).unwrap();
+    }
+
+    /// glUseProgram
+    pub fn use_program(&self) -> WebGLResult<()> {
+        if !self.linked.get() {
+            return Err(WebGLError::InvalidOperation);
         }
 
         self.renderer.send(CanvasMsg::WebGL(WebGLCommand::UseProgram(self.id))).unwrap();