Comment # 2 on bug 93970 from Ilia Mirkin

Try running with force_glsl_extensions_warn=true in the environment. Looks like
the shader tries to use some advanced functionality without enabling the
extensions. It checks with #ifdef GL_ARB_gpu_shader5, without enabling the ext,
which is unfortunate. My understanding is that the preprocessor macros are set
to 1 even when the ext isn't enabled with #extension.