glsl: add compilation errors for attribute and varying qualifiers
These qualifiers were removed in GLSL 4.20 and GLSL ES 3.00. For desktop GL they are now only allowed if the shader is declared as a compatibility shader.
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These qualifiers were removed in GLSL 4.20 and GLSL ES 3.00. For desktop GL they are now only allowed if the shader is declared as a compatibility shader.