BView: add methods for affine translation/scaling/rotation

* BView::TranslateBy(), BView::ScaleBy() and BView::RotateBy()
  allow to conveniently modify the current affine transformation.
  This makes it unnecessary to first read the current transform,
  modify it, and then set it again.
  Uses the new Pre...() methods of BAffineTransform.

* Also, remove setting the transform "through" to the BView even
  while recording a BPicture, as this now results in transforms
  being applied more than once.

headers/os/interface/View.h

@@ -300,6 +300,9 @@ public:
 								// more powerful alternative.
 			void				SetTransform(BAffineTransform transform);
 			BAffineTransform	Transform() const;
+			void				TranslateBy(double x, double y);
+			void				ScaleBy(double x, double y);
+			void				RotateBy(double angleRadians);
 
 			void				PushState();
 			void				PopState();