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 <[email protected]>
Date:   Thu Aug 26 18:17:54 2010 +0200

    mesa/st: respect fragment clamping in st_DrawPixels

commit 546a31e42cad459d7a7a10ebf77fc5ffcf89e9b8
Author: Luca Barbieri <[email protected]>
Date:   Thu Aug 26 18:17:28 2010 +0200

    mesa/st: support fragment and vertex color clamping

commit c406514a1fbee6891da4cf9ac3eebe4e4407ec13
Author: Luca Barbieri <[email protected]>
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 <[email protected]>
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 <[email protected]>
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 <[email protected]>
Date:   Thu Aug 26 18:18:25 2010 +0200

    gallium: add color clamping to the interface

1 files changed, 7 insertions, 0 deletions

diff --git a/src/mesa/state_tracker/st_extensions.c b/src/mesa/state_tracker/st_extensions.c
index bbcae3b02e9..89bb03046c1 100644
--- a/src/mesa/state_tracker/st_extensions.c
+++ b/src/mesa/state_tracker/st_extensions.c
@@ -508,6 +508,13 @@ void st_init_extensions(struct st_context *st)
       ctx->Extensions.ARB_depth_clamp = GL_TRUE;
    }
 
+   /* this extension does not actually require support of floating point
+    * render targets, just clamping controls
+    */
+   if(screen->get_param(screen, PIPE_CAP_FRAGMENT_COLOR_CLAMP_CONTROL) &&
+      screen->get_param(screen, PIPE_CAP_VERTEX_COLOR_CLAMP_CONTROL))
+      ctx->Extensions.ARB_color_buffer_float = GL_TRUE;
+
    if (screen->get_param(screen, PIPE_CAP_SHADER_STENCIL_EXPORT)) {
       ctx->Extensions.ARB_shader_stencil_export = GL_TRUE;
    }