fix data type inconsistency in spritematrix

[caogtaa]

Before fix:

[david-pfx]

Do you have a repro to show what the problem is?

[increpare]

(Seems a sensible enough fix to me given the context of the code - I imagine even the basic sokoban example with the crate as defined above will exhibit this - apologies for not merging it yet - busy trying to prep for the release of a new game and haven't had time to propertly test/vet it [it's also not a critical error, just good hygeine])