gpu: Pixel-align the blur rectangle

Fixes blurriness in shadows.

Not sure to do a proper test for this feature. Usually proper pixel
alignment is tested by drawing a crips line and checking that it is
indeed crisp. But we are testing the blur operation here...

Fixes #6380

gsk/gpu/gskgpunodeprocessor.c

@@ -1015,6 +1015,8 @@ gsk_gpu_node_processor_blur_op (GskGpuNodeProcessor       *self,
   if (!gsk_rect_intersection (rect, &clip_rect, &intermediate_rect))
     return;
 
+  rect_round_to_pixels (&intermediate_rect, &self->scale, &self->offset, &intermediate_rect);
+
   intermediate = gsk_gpu_node_processor_init_draw (&other,
                                                    self->frame,
                                                    source_depth,