New Display::Mode struct, a few new methods, and fixes.

- Changed Display::Resolution to Display::Mode, with other signatures
- Added Display::setMouseCursorVisible() method
- Added some auxiliary methods to fgeal::Rectangle
- Fixed transparency of newly created Image objects with SDL 2.0 adapter
- Fixed "shaking" when drawing using drawScaledRegion() with SDL 1.2
adapter
- Fixed backbuffer not being resized at display resize with Allegro 4
adapter
- Some refactoring