* Add MakeRectangle helper to convert from SDL_Rect
* Add Rectangle::inset method for shrinking a rectangle
Turns out some of the other use cases I though this could apply to were actually doing something based on a fixed region
* Simplify initialisation of settings menu rects
* Use appropriate types for size constants in control.cpp
* Declare constexpr value for iterating over the cells in a stash grid
* Use appropriate type for UIRectangle dimensions
Makes `CelSprite` unowned and adds a new `OwnedCelSprite` class for
owned sprites.
This clarifies ownership and makes the code cleaner in a number of
places.
Additionally, because the `CelSprite` class is now tiny (1 less
pointer), we can pass it by-value instead of by-reference, removing a
pointer indirection in the rendering functions.
ephphatha