Room editor fixes and improvements.

[VladiStep]

1. All room objects (game object instances, tile instances, backgrounds, etc.) with textures that have non-zero target coordinates are now displayed properly (TargetX and TargetY are used now).
2. Added XOffset and YOffset properties to Background, SpriteInstance and LayerBackgroundData.
3. Frame index of GameObject and SpriteInstance is now used in much more places: objects render size, GenerateSpriteCache(), XOffset and YOffset.
4. Frame index UI field doesn't limit the value to the bounds.
5. Frame index wrappers are changed from SafeImageIndex/SafeFrameIndex to WrappedImageIndex/WrappedFrameIndex - now it's a read-only property that wraps frame index value to a valid array index.
   For example - if sprite has 3 frames, and image index is 4, then it will return 1 - second frame.
6. Removed redundant null checks on sprite Textures (Textures can't be null).
7. Renamed some private fields so that they comply with the naming convention (_PrivateBackingField -> _privateBackingField).
8. Fixed tiles texture cropping.
9. Fixed room objects pasting (with Ctrl+V).
10. Extracted common code of PaintObjects() and Command_Paste(), and made new AddObjectCopy() method from it.
11. Made room object pasting error messages more polite.
12. Simplified some new T() expressions (new ClassWithLongName() -> new()).
13. Set proper default values for SpriteInstance to simplify new instance creation.