Fix sprite transparency to check for non-zero bits

[jtothebell]

Pico-8 seems to only care that the transparency nibble in the draw palette is non-zero, but before this fix zepto8 was checking that it was exactly 1. I don't see this difference explicitly spelled out in the official docs, but the wiki does say "non-zero" (https://pico-8.fandom.com/wiki/Memory#Draw_state).

This seems to only affect carts that are pokeing the draw state. One example that is fixed is Dank Tomb, where the green background of the character sprite was visible in zepto8, but is not after this fix.