arb_copy_image/formats.c invokes undefined behavior

This test uses piglit_multisample_texture to initialize textures in the multisample case. However, this invokes undefined behavior when used for integer formats. From the GL spec:

If the values written by the fragment shader do not match the format(s) of the corresponding color buffer(s), the result is undefined.

The problem (as suggested by the quote above) is that piglit_multisample_texture doesn't use the correct type for the fragment shader output, which makes the written values undefined.

/cc @gfxstrand ... You wrote this test originally, and the problem seems to go all the way back to that version. Thoughts?