Computed size field in the Sprite structure

## What problem does this solve or what need does it fill?

Getting sprite sizes is not ergonomic nor productive at the moment.

## What solution would you like?

Adding a `computed_size: Option` or similar to the `Sprite` struct. That field will be updated from the sprite engine whenever it knows the size and image of the Sprite.

## What alternative(s) have you considered?

The alternative is every time we need the size of a sprite (or an image associated to one), to request the asset server, the image handle, checking that the image is loaded and then accessing to its size and potentially creating a resource or a component in the entity with the size for a system or systems to access.