gallium: add PIPE_CAP_MAX_WINDOW_RECTANGLES to all drivers

This says how many window rectangles are supported by the
implementation, although it may not exceed PIPE_MAX_WINDOW_RECTANGLES.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/drivers/softpipe/sp_screen.c

@@ -276,6 +276,7 @@ softpipe_get_param(struct pipe_screen *screen, enum pipe_cap param)
    case PIPE_CAP_ROBUST_BUFFER_ACCESS_BEHAVIOR:
    case PIPE_CAP_PRIMITIVE_RESTART_FOR_PATCHES:
    case PIPE_CAP_TGSI_VOTE:
+   case PIPE_CAP_MAX_WINDOW_RECTANGLES:
       return 0;
    case PIPE_CAP_SHADER_BUFFER_OFFSET_ALIGNMENT:
       return 4;