webgl: Refactor implementation to move logic inside the DOM interfaces

This improves the encapsulation and consistency in our WebGL
implementation.

Also allows to implement new methods such as `getShaderSource()`.

It will also allow us to use `delete()` in the destructors of them (note
that we will want to keep track of them from the context).

components/canvas_traits/lib.rs

@@ -107,9 +107,9 @@ pub enum CanvasWebGLMsg {
     BindTexture(u32, u32),
     DrawArrays(u32, i32, i32),
     EnableVertexAttribArray(u32),
-    GetAttribLocation(u32, String, Sender<i32>),
-    GetShaderInfoLog(u32, Sender<String>),
-    GetShaderParameter(u32, u32, Sender<i32>),
+    GetShaderInfoLog(u32, Sender<Option<String>>),
+    GetShaderParameter(u32, u32, Sender<WebGLShaderParameter>),
+    GetAttribLocation(u32, String, Sender<Option<i32>>),
     GetUniformLocation(u32, String, Sender<Option<i32>>),
     LinkProgram(u32),
     ShaderSource(u32, String),
@@ -121,6 +121,24 @@ pub enum CanvasWebGLMsg {
     DrawingBufferHeight(Sender<i32>),
 }
 
+#[derive(Clone)]
+pub enum WebGLError {
+    InvalidEnum,
+    InvalidOperation,
+    InvalidValue,
+    OutOfMemory,
+    ContextLost,
+}
+
+pub type WebGLResult<T> = Result<T, WebGLError>;
+
+#[derive(Clone)]
+pub enum WebGLShaderParameter {
+    Int(i32),
+    Bool(bool),
+    Invalid,
+}
+
 #[derive(Clone)]
 pub enum CanvasCommonMsg {
     Close,