Determining visible entities isn't quite right

[phrohdoh]

As stated in the commit message of 040ccc3c1ea9bc797a906ce0c919dc594d36f54b:

It works for the most part, but units do disappear on the edge of the screen occasionally.

This will just give us an issue to close once this is resolved properly.

I can only get this to happen with:

• The bottom two corners of the viewport
• Elevated entities

This GIF was taken using scenario mu400f_1.

[angered-ghandi]

Terrain is now also affected by this, likely for the same (or similar) reason.

[phrohdoh]

Ok so terrain will be filtered out if...

• Elevation > 1:
  • Anywhere along the bottom border
• Elevation == 0:
  • Only in the bottom two corners

[angered-ghandi]

Just realized this entire region will be covered up by the UI anyway. May not need to fix it.