[Piglit] Patch adding test for fragment and vertex texturing in the same program

[Julian Adams]

I reported a bug a couple of weeks ago where sampling textures in both
the vertex and fragment shaders samples the incorrect texture in the
vertex shader: https://bugs.freedesktop.org/show_bug.cgi?id=35849.
There's an attached piglit patch that demonstrates the issue. Please
review and apply.

For me the test fails using the r600g driver. It would be useful to
know if the test works for other hardware and drivers to help narrow
down where the bug is.

Thanks,

Jools