Some improvements to grid and navigation

Some improvements to grid and navigation

Extended viewable area past map border;
Made scrollbars non-discrete (per-pixel instead of per-tile);
Bound grid to map origin instead of viewport, same for cursor snap;
Added speed modifier for WASD navigation (hold Shift);
Changed and enforced possible grid sizes to 4-2048;
Fixed related crawled-up bugs, changed code logic here and there, made other stuff I forgot to mention.