Add the ability to mod mouse cursors

This supports cursors for:
* Hovering over map events and loot
* Hovering over living enemies
* Hovering over NPCs

As well as a normal cursor.
Allow setting offsets for modded mouse cursors
Merge pull request #979 from dorkster/fixes

Reload icons after recreating render context
Merge pull request #977 from dorkster/mouse_cursor

Context sensitive mouse cursor graphics
Reinitialize CursorManager when changing mods or resolution