Investigate if GetTexParameter should fail if 0 is returned.

[gootorov]

Currently GetTexParameter fails if 0 is returned, but none of the specs specify that.
Specs:
WebGL
GLES

servo/components/canvas/webgl_thread.rs

Lines 1075 to 1077 in 730bd5e

	let parameter = gl.get_tex_parameter_iv(target, pname);
	if parameter == 0 {
	Ok(WebGLParameter::Invalid)

servo/components/script/dom/webglrenderingcontext.rs

Lines 1362 to 1364 in 730bd5e

	WebGLParameter::Invalid => {
	self.webgl_error(InvalidEnum);
	NullValue()

[fnune]

This is not the case anymore:

    fn get_tex_parameter(gl: &gl::Gl,
                       target: u32,
                       pname: u32,
                       chan: WebGLSender<i32> ) {
        let result = gl.get_tex_parameter_iv(target, pname);
        chan.send(result).unwrap();
    }

https://github.com/servo/servo/blob/master/components/canvas/webgl_thread.rs#L1084-L1090
I think?

[gootorov]

It was refactored, but the check was left intact:

servo/components/script/dom/webglrenderingcontext.rs

Lines 1408 to 1414 in e06f0d3

	match receiver.recv().unwrap() {
	value if value != 0 => Int32Value(value),
	_ => {
	self.webgl_error(InvalidEnum);
	NullValue()
	}
	}

If it's not needed, the whole thing can be simplified to Int32Value(receiver.recv().unwrap()).