webgl: Refactor texture validations to take advantage of rust type system

This commit introduces the `WebGLValidator` trait, and uses it for multiple
validations in the texture-related WebGL code, to move that logic out of the
already bloated `webglrenderingcontext.rs` file.

It also creates a type-safe wrapper for some WebGL types, removing all the
`unreachable!`s there, and introduces a macro for generating them conveniently.

This partially addresses #10693, pending refactor more code to use this
infrastructure, and (possibly?) introducing an `AsGLError` trait for the errors
to make the error handling happen in `WebGLContext`.

components/script/dom/webgl_validations/mod.rs

@@ -0,0 +1,13 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+pub trait WebGLValidator {
+    type ValidatedOutput;
+    type Error: ::std::error::Error;
+
+    fn validate(self) -> Result<Self::ValidatedOutput, Self::Error>;
+}
+
+pub mod tex_image_2d;
+pub mod types;