Scrolling glitches when moving across tilemap

[djhworld]

There are some issues with the drawing code that I think are connected to scrolling, like for example when you scroll across the map in Super Mario Land 2 it glitches when Scroll Y wraps round to from 0 down to 255

[djhworld]

fixed in 357a70d

the calculation to work out the tilemap offset wasn't wrapping around. The gameboy scrolling logic wraps around if the scroll area wraps off the screen