draw: implement vertex color clamping, and disable SSE and PPC paths

draw: implement vertex color clamping, and disable SSE and PPC paths

(some little changes by Marek Olšák)

Squashed commit of the following:

commit 737c0c6b7d591ac0fc969a7590e1691eeef0ce5e
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Fri Aug 27 02:13:57 2010 +0200

    draw: disable SSE and PPC paths (use LLVM instead)

    These paths don't support vertex clamping, and are anyway
    obsoleted by LLVM.

    If you want to re-enable them, add vertex clamping and test that it
    works with the ARB_color_buffer_float piglit tests.

commit fed3486a7ca0683b403913604a26ee49a3ef48c7
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Thu Aug 26 18:27:38 2010 +0200

    draw_llvm: respect vertex color clamp

commit ef0efe9f3d1d0f9b40ebab78940491d2154277a9
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Thu Aug 26 18:26:43 2010 +0200

    draw: respect vertex clamping in interpreter path