compositor: Fix cursor redraw on scale change

[Drakulix]

So because we are allocating a new buffer for cursor planes, the config.dst of the previous plane state might not actually represent the size of the cursor element, but simply the size of the cursor-plane (or the nearest size hint).

That might stay the same, if the cursor is simply rescaled, resulting in our tests failing to detect, that we need to redraw. This in particular happens with MemoryRenderElements + RescaleRenderElements, where neither the Id nor the commit_counter changes. So we have to remember the element size.

(Or we make src/dst actually use the cursor size, without re-allocating the buffer of course, but I am not sure, if compatibility for that is so great. Generally cursor planes don't seem very capable.)