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`.
