NEXXT 2.1.1 - fixed broken 8x16 sprite mode, improved it too

Phew, another post-release bug reported and fixed. Hopefully that'll be the last in a while. I'm hoping to take a breather. 

Fixes a bug where 8x16 sprites would not display at all under certain conditions. 

In addition, selecting or dropping an 8x16 sprite now also selects the 2 tiles it refers to on the tileset. This works regardless if you're viewing the tileset normally (as ordered in NES PPU memory), or in the special "8x16 viewmode" that makes editing sprite contents easier.