Add a new test for a structure uniform splitting bug with samplers

Kenneth Graunke requested to merge kwg/piglit:struct-bug into master

Michael Fiano discovered an i965 crash in Mesa 19.0 when a uniform structure contains samplers. We were splitting the structure into separate uniforms but botching the locations, leaving them set to 0, then reading the wrong uniform storage and seeing the VS's matrix.

This test was reduced from his (MIT-licensed) application's shaders.

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