Fix editor using the game's sprite sheet for its own assets

[jjant]

Description

Currently, if you run any game: cargo run --bin celeste, the editor will look wrong:

This is because we're using the same pico8.spr(...) functions for rendering editor sprites, but these use the current game's sprite sheet.

To fix this we want to load the editor sprite sheet separately and use Pico8::spr_from instead: pico8.spr_from(editor_sprite_sheet, ...).

We probably only want to tackle this after #16 is done, as the current setup allows editing editor sprites directly from the editor (by running cargo run) which is useful.