Simpler dashed line calculations.
Move bounding box drawing routine into Minimap.
Remove invalid hex function.

It's not appropriate to have this test anymore because the RandomMap
terrain grid allows negative hexes.
Beginnings of mouse handling.

Moved rectangle functions to sdl_helper.  These might become their own
thing at some point.
Add click and drag on the minimap bounding box.
New function to limit a number to given bounds.

Useful for making sure we stay within map boundaries when the user drags
the mouse around.

Also, removed the initial pan of the map now that the user can move it.
Added Herb Sutter's make_unique.
Refactored and corrected mouse handling code.