Roblox maps some rbxasset:// names to downloads and opens no local file for
them. For each file under content/ in the overlay folder, change its name in
the loaded libroblox so Roblox opens the local file.
Idea from @dim-ghub in #20.
Adds textures/ with the crosshair PNG named by its ETC2 hash and a README
covering install, naming and how to dump hashes for other textures. Nothing
installs or loads it; copying into ~/.config/mocktail/textures is opt-in.