Theme fallback parts outside of the original sheet size break

Since bc699e6ebae2bfd838e81d4be4c3bdc282cddf4c expanded the default theme's sheet.png, any theme that has a sheet of the original size will not fall back correctly to the previous part, because the system doesn't check for these changes, and everything happens with m_sheet being the original one so it's not a trivial fix to just check if it's in bounds when we can't tell afterwards what's a fallback piece and what isn't, it'll most likely need a refactor.

Example: