Improve 2d snapping

Partially revert change allowing sprite get_rect snapping to be controlled by `pixel_snap` again rather than `transform_snap` (to prevent breaking compatibility). Adds a final `use_camera_snap` project setting to allow snapping viewports as in reduz original PR.

core/engine.h

@@ -60,6 +60,7 @@ private:
 	float _time_scale;
 	bool _pixel_snap;
 	bool _snap_2d_transforms;
+	bool _snap_2d_viewports;
 	uint64_t _physics_frames;
 	float _physics_interpolation_fraction;
 
@@ -109,6 +110,7 @@ public:
 
 	_FORCE_INLINE_ bool get_use_pixel_snap() const { return _pixel_snap; }
 	bool get_snap_2d_transforms() const { return _snap_2d_transforms; }
+	bool get_snap_2d_viewports() const { return _snap_2d_viewports; }
 
 #ifdef TOOLS_ENABLED
 	_FORCE_INLINE_ void set_editor_hint(bool p_enabled) { editor_hint = p_enabled; }