gallium: implement clamping controls (ARB_color_buffer_float)

gallium: implement clamping controls (ARB_color_buffer_float)

BTW this changes the gallium interface.
Some rather cosmetic changes by Marek.

Squashed commit of the following:

commit 513b37d484f0318311e84bb86ed4c93cdff71f13
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Thu Aug 26 18:17:54 2010 +0200

    mesa/st: respect fragment clamping in st_DrawPixels

commit 546a31e42cad459d7a7a10ebf77fc5ffcf89e9b8
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Thu Aug 26 18:17:28 2010 +0200

    mesa/st: support fragment and vertex color clamping

commit c406514a1fbee6891da4cf9ac3eebe4e4407ec13
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Tue Aug 24 21:56:37 2010 +0200

    mesa/st: expose ARB_color_buffer_float if unclamping is supported

commit d0c5ea11b6f75f3da2f4ca989115f150ebc7cf8d
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Thu Aug 26 17:53:41 2010 +0200

    mesa/st: use unclamped colors

    This assumes that Gallium is to be interpreted as given drivers the
    responsibility to clamp these colors if necessary.

commit aef5c3c6be6edd076e955e37c80905bc447f8a82
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Thu Aug 26 18:12:34 2010 +0200

    mesa, mesa/st: handle read color clamping properly

    We set IMAGE_CLAMP_BIT in the caller based on _ClampReadColor, where
    the operation mandates it. (see the removed XXX comment. -Marek)

    TODO: did I get the set of operations mandating it right?

commit 76bdfcfe3ff4145a1818e6cb6e227b730a5f12d8
Author: Luca Barbieri <luca@luca-barbieri.com>
Date:   Thu Aug 26 18:18:25 2010 +0200

    gallium: add color clamping to the interface