softpipe: add a better fake implementation of fences

The flush function, when asked for, should not return a NULL fence.

NULL can only be returned if fences are not implemented, and st/mesa
doesn't call any of the fence functions if it receives a NULL fence
(because some drivers don't even set the fence hooks).

ARB_sync is exposed if fence_finish is set.

src/gallium/drivers/softpipe/sp_fence.c

@@ -36,8 +36,7 @@ softpipe_fence_reference(struct pipe_screen *screen,
                          struct pipe_fence_handle **ptr,
                          struct pipe_fence_handle *fence)
 {
-   assert(!*ptr);
-   assert(!fence);
+   *ptr = fence;
 }
 
 
@@ -45,7 +44,7 @@ static boolean
 softpipe_fence_signalled(struct pipe_screen *screen,
                          struct pipe_fence_handle *fence)
 {
-   assert(!fence);
+   assert(fence);
    return TRUE;
 }
 
@@ -55,7 +54,7 @@ softpipe_fence_finish(struct pipe_screen *screen,
                       struct pipe_fence_handle *fence,
                       uint64_t timeout)
 {
-   assert(!fence);
+   assert(fence);
    return TRUE;
 }
 

src/gallium/drivers/softpipe/sp_flush.c

@@ -96,9 +96,9 @@ softpipe_flush( struct pipe_context *pipe,
       ++frame_no;
    }
 #endif
-   
+
    if (fence)
-      *fence = NULL;
+      *fence = (void*)(intptr_t)1;
 }
 
 void