webgl: Add shader validation and translation

This commit adds angle-based validation and translation to WebGL
shaders.

The changes to the tex_image_2d test is neccessary (it was not valid
GLES 2.0 shader language).

components/script/lib.rs

@@ -78,6 +78,7 @@ extern crate url;
 extern crate uuid;
 extern crate string_cache;
 extern crate offscreen_gl_context;
+extern crate angle;
 extern crate tendril;
 
 pub mod cors;