…tion

This is a large-ish refactor of the Texture2D code, but it should be
easier to read and of course more correct.

I tried to annotate every error condition with a spec paragraph.

greater than 1

This makes it more correct, since we don't blindly send the Link
command. It's not observable though.

It's broken for uniform arrays, since uniform.id() stops being the index
then.

We need to add a more complex integration with angle for this to ever
be correct.

Unfortunately the ANGLE part that we should touch is C++, and it has
destructors, so we need to hook destructors there, and I can't do it
right now.

This makes the test bufferDataBadArgs pass appropiately.

You can fill a level > 0 as long as the width and height values are
power of two, so the previous test was bogus.

…face

Also refactor a bit the code, and remove the unused `is_initialized`
flag.

It now passes consistently.

This tests previously "passed", because they used the texImage2D
overload we're adding now.

Since that overload previously caused a TypeError, these tests were
never run in the first place.