BColorControl: Remove fOffscreenView pointer

...and rename fBitmap to fOffscreenBitmap to make it more clear what it is.

We don't need to save a pointer to both the offscreen bitmap and
the offscreen view, just the bitmap. We can access the view by calling
fOffscreenBitmap->ChildAt((int32)0). This gives us back a _reserved private
variable slot.

In the (unlikely) case that _InitData() is called with offscreen = false but
the fOffscreenBitmap is not NULL, delete fOffscreenBitmap before
setting it to NULL so that memory is not leaked.

headers/os/interface/ColorControl.h

@@ -125,12 +125,11 @@ private:
 			BTextControl*		fGreenText;
 			BTextControl*		fBlueText;
 
-			BBitmap*			fBitmap;
-			BView*				fOffscreenView;
+			BBitmap*			fOffscreenBitmap;
 
 			int16				fFocusedRamp;
 			int16				fClickedRamp;
-			uint32				_reserved[2];
+			uint32				_reserved[3];
 };
 
 inline void