We were checking to see if it was too big to be level 0, but we really
want to see if it's too big to be the given level.
This was the last remaining failure in texture-size-limit.html.
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`.
