Improved package rotation and slotting

UPDATES

• Obects now rotate around the cursor instead of around the center of the image.
• Package slotting in the blueprint grid is much more accurate and the packages smoothly snaps to the grid.
• Items lost by being moved off of the screen will be smoothly moved back to their starting position.
• Initial support for irregularly shaped packages (think tetris), but none are in game yet.
• Item shadows behave as expected during rotation.