st/mesa: do vertex and fragment color clamping in shaders

For ARB_color_buffer_float. Most hardware can't do it and st/mesa is
the perfect place for a fallback.
The exceptions are:
- r500 (vertex clamp only)
- nv50 (both)
- nvc0 (both)
- softpipe (both)

We also have to take into account that r300 can do CLAMPED vertex colors only,
while r600 can do UNCLAMPED vertex colors only. The difference can be expressed
with the two new CAPs.

src/gallium/drivers/softpipe/sp_screen.c

@@ -128,7 +128,9 @@ softpipe_get_param(struct pipe_screen *screen, enum pipe_cap param)
       return 7;
    case PIPE_CAP_CONDITIONAL_RENDER:
       return 1;
-   case PIPE_CAP_FRAGMENT_COLOR_CLAMP_CONTROL:
+   case PIPE_CAP_FRAGMENT_COLOR_CLAMPED:
+   case PIPE_CAP_VERTEX_COLOR_UNCLAMPED: /* draw module */
+   case PIPE_CAP_VERTEX_COLOR_CLAMPED: /* draw module */
       return 1;
    default:
       return 0;