webgl: Implement multiple calls and improve error detection

This commit implements WebGL's:
 * cullFace
 * frontFace
 * enable
 * disable
 * depthMask
 * colorMask
 * clearDepth
 * clearStencil
 * depthFunc
 * depthRange
 * hint
 * lineWidth
 * pixelStorei
 * polygonOffset
 * texParameteri
 * texParameterf
 * texImage2D (partially)

It inlines a lot of OpenGL calls to keep the file
`components/canvas/webgl_paint_task.rs` as small as possible while
keeping readability.

It also improves error detection on previous calls, and sets node damage
on the canvas in the drawing calls.

It adds a `TexImage2D` reftest, even though it's not enabled because:
 * WebGL paints the image when it loads (asynchronously), so the reftest doesn't wait for it and it finishes early
 * If we change the source for the base64 src of the image it works as expected in non-headless mode, but the test harness locks

components/script/dom/webglbuffer.rs

@@ -9,7 +9,7 @@ use dom::bindings::js::Root;
 use dom::bindings::utils::reflect_dom_object;
 use dom::webglobject::WebGLObject;
 
-use canvas_traits::{CanvasMsg, CanvasWebGLMsg};
+use canvas_traits::{CanvasMsg, CanvasWebGLMsg, WebGLError, WebGLResult};
 use ipc_channel::ipc::{self, IpcSender};
 use std::cell::Cell;
 
@@ -18,6 +18,8 @@ use std::cell::Cell;
 pub struct WebGLBuffer {
     webgl_object: WebGLObject,
     id: u32,
+    /// The target to which this buffer was bound the first time
+    target: Cell<Option<u32>>,
     is_deleted: Cell<bool>,
     #[ignore_heap_size_of = "Defined in ipc-channel"]
     renderer: IpcSender<CanvasMsg>,
@@ -28,6 +30,7 @@ impl WebGLBuffer {
         WebGLBuffer {
             webgl_object: WebGLObject::new_inherited(),
             id: id,
+            target: Cell::new(None),
             is_deleted: Cell::new(false),
             renderer: renderer,
         }
@@ -49,7 +52,7 @@ impl WebGLBuffer {
 
 pub trait WebGLBufferHelpers {
     fn id(self) -> u32;
-    fn bind(self, target: u32);
+    fn bind(self, target: u32) -> WebGLResult<()>;
     fn delete(self);
 }
 
@@ -58,8 +61,18 @@ impl<'a> WebGLBufferHelpers for &'a WebGLBuffer {
         self.id
     }
 
-    fn bind(self, target: u32) {
+    // NB: Only valid buffer targets come here
+    fn bind(self, target: u32) -> WebGLResult<()> {
+        if let Some(previous_target) = self.target.get() {
+            if target != previous_target {
+                return Err(WebGLError::InvalidOperation);
+            }
+        } else {
+            self.target.set(Some(target));
+        }
         self.renderer.send(CanvasMsg::WebGL(CanvasWebGLMsg::BindBuffer(target, self.id))).unwrap();
+
+        Ok(())
     }
 
     fn delete(self) {