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/Cargo.toml

@@ -55,6 +55,10 @@ features = ["query_encoding", "serde_serialization"]
 [dependencies.offscreen_gl_context]
 git = "https://github.com/ecoal95/rust-offscreen-rendering-context"
 
+[dependencies.angle]
+git = "https://github.com/ecoal95/angle"
+branch = "servo"
+
 [dependencies.cssparser]
 version = "0.3"
 features = [ "serde-serialization" ]