[3DS] add background2 overdraw

[ds-sloth]

In multires on 3DS, the vScreen bounds sometimes extend past the section bounds. This is good because it allows the 3D effect to work, but there are some issues:

• at the section bounds, the draw code for some background2s only draws areas within the section bounds.
• for 800px-wide sections, the vScreen bounds are stuck at 800px.

For this issue:

• [x] For 800px-wide sections, allow the vScreen bounds to extend to 840px (the overdraw view used for 3DS)
• [ ] Allow background2 drawing to extend past the section bounds

[ds-sloth]

I resolved the first case. The second one will happen after #463