Fix horizontal offset in high-resolution offset-by-tile mode

[Morilli]

• closes Mode 6 emulation is inaccurate (horizontal scrolling not applied correctly) #384

The intuition behind this fix is that if we're in hires mode (bg mode 5 or 6), the background's hoffset needs to be doubled. In ppu-fast, I've just added an additional left-shift to double this value and in the accurate ppu we just do one less shift right (alternatively, we could left-shift both io.hoffset and characterIndex individually).

In ppu-fast, I've applied an additional fix to the "column - 1" logic by replacing 8 with 1 << tileWidth, which is the same for 8x8 tiles and doubled (16) for 16x[8/16] tiles. This seems to be visually accurate based on the opt-big-map6.sfc testrom provided in the original issue.

I believe this is a problem exclusive to bg mode 6 because that's the only hires+offset-by-tile mode.