cursor-renderer-native: Floor the cursor position instead of rounding

Which eliminates the 1px jitter that was visible when dragging windows,
and eliminates the flickering that was visible when pushing the cursor
against the right/bottom edges of the screen.

src/backends/native/meta-cursor-renderer-native.c

@@ -366,8 +366,8 @@ update_monitor_crtc_cursor (MetaMonitor         *monitor,
                        scaled_crtc_rect.origin.y) * scale;
       drmModeMoveCursor (kms_fd,
                          crtc->crtc_id,
-                         roundf (crtc_cursor_x),
-                         roundf (crtc_cursor_y));
+                         floorf (crtc_cursor_x),
+                         floorf (crtc_cursor_y));
 
       data->out_painted = data->out_painted || TRUE;
     }