Fix mouse click based cursor actions

Previously they did not properly take into
account the y co-ordinate offset caused by the segment's Z-axis
size. This is due in large part to confusion caused by
the two different conventions used for segment's z-positions.

The global `ssState` variable has a convention that Z is 0 on
the top most layer of the segment, while the `segment->segState`
variable uses the convention that Z is zero for one below the
bottom layer.
Fix positioning of debug cursor

Because the `segment->segState.dfCursor` is
always in game-based co-ordinates unconditionally
convert it to segment based co-ordinates before
displaying it.
Fix click offset rounding errors.

Clicks were resolving to tiles that were off by one due
to roundoff errors in the `ScreenToPoint` function.
Using float math internally gives gives correct results
to +/- 1 pixel.

Fixes #19.
Fix click based cursors in Fortress Mode

A few bugs related to the one tile border cause
clicks to target the wrong tile. This fixes that.
Merge pull request #20 from KevinCathcart/CursorFixes

Cursor fixes