gl_device: Fix TestVariableAoffi test

This test is intended to be invalid GLSL, but it was being invalid in
two points instead of one. The intention is to use a non-immediate
parameter in a textureOffset like function.

The problem is that this shader was being compiled as a separable
shader object and the text was writting to gl_Position without a
redeclaration, being invalid GLSL.

Address that issue by using a user-defined output attribute.