shader: correct swizzles from TGSI_SEMANTIC_SAMPLEPOS

TGSI defines TGSI_SEMANTIC_SAMPLEPOS as a 4-component vector with Z and W set to 0. However, if we ever try to reach thez and w compoents, we run into trouble because this is implemented as a swizzle on gl_SamplePosition instead. And gl_SamplePosition only has x and y.

It seems that recent versions of mesa does just this, causing shader-compilation errors during dEQP tests.

This fixes regressions in the following tests:

• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.correctness.singlesample_texture
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.correctness.multisample_texture_1
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.correctness.multisample_texture_2
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.correctness.multisample_texture_4
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.correctness.multisample_texture_8
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.correctness.singlesample_rbo
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.correctness.multisample_rbo_1
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.correctness.multisample_rbo_2
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.correctness.multisample_rbo_4
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.correctness.multisample_rbo_8
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.distribution.multisample_texture_1
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.distribution.multisample_texture_2
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.distribution.multisample_texture_4
• dEQP-GLES31.functional.shaders.sample_variables.sample_pos.distribution.multisample_texture_8
• dEQP-GLES31.functional.shaders.multisample_interpolation.interpolate_at_offset.at_sample_position.default_framebuffer

Signed-off-by: Erik Faye-Lund erik.faye-lund@collabora.com