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.
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