app_server: add method to shift alpha masks

* Allow shifting the offset of alpha masks without changing the size.
  Ideally, we only need to reattach the buffer in the shifted
  position, saving the work of reallocating and redrawing the mask
  picture. Needed for layers support.

src/servers/app/drawing/AlphaMask.cpp

@@ -89,6 +89,25 @@ AlphaMask::Update(BRect bounds, BPoint offset)
 }
 
 
+BPoint
+AlphaMask::Update(BPoint offset)
+{
+	BPoint oldOffset = fViewOffset;
+	fViewOffset = offset;
+
+	if (oldOffset == fCachedOffset && fCachedBitmap != NULL) {
+		// No need to redraw the picture when only the offset is shifted
+		fCachedOffset = offset;
+		_AttachMaskToBuffer();
+	}
+
+	if (fPreviousMask != NULL)
+		fPreviousMask->Update(offset);
+
+	return oldOffset;
+}
+
+
 void
 AlphaMask::SetPrevious(AlphaMask* mask)
 {

src/servers/app/drawing/AlphaMask.h

@@ -27,6 +27,7 @@ public:
 								~AlphaMask();
 
 			void				Update(BRect bounds, BPoint offset);
+			BPoint				Update(BPoint offset);
 
 			void				SetPrevious(AlphaMask* mask);