app_server: add new BView layers API

* Add new methods
    BView::BeginLayer(uint8 opacity)
    BView::EndLayer()

* All drawing between begin and end of a layer is redirected onto an
  intermediate bitmap. When ending the layer, this bitmap is
  composited onto the view with the opacity given when the layer was
  started.

* Layers can be nested arbitrarily and will be blended onto each
  other in order. There can also be any arbitrary interleaving of
  layer begin/end and drawing operations.

* Internally, drawing commands are redirected into a BPicture between
  BeginLayer and EndLayer (but client code need not know or care
  about this). Client code can also start/end other BPictures while
  inside a layer.

* Uses the PictureBoundingBoxPlayer to determine the size of the
  layer bitmap before allocating and drawing into it, so it does not
  allocate more memory than necessary and -- more importantly -- it
  will not alpha-composite more pixels than necessary.

* Drawing mode is always set to B_OP_ALPHA, blend mode to
  (B_PIXEL_ALPHA, B_ALPHA_COMPOSITE) while inside layers. This is
  necessary for (a) correct compositing output and (b) for
  redirection of drawing into the intermediate bitmap, which uses the
  renderer_region offset (in B_OP_COPY, the Painter does not use the
  AGG renderer methods, it directly accesses the pixel data. This
  would access out-of-bounds without the offset, so B_OP_COPY cannot
  be allowed.)
  To ensure these modes aren't changed, BView::SetDrawingMode()
  and BView::SetBlendingMode() are ignored while inside a layer.

* The main motivation behind this new API is WebKit, which internally
  expects such a layers functionality to be present. A performant and
  reusable implementation of this functionality can only be done
  server-side in app_server.

headers/private/interface/PictureDataWriter.h

@@ -1,9 +1,10 @@
 /*
- * Copyright 2006-2007 Haiku, Inc. All rights reserved.
+ * Copyright 2006-2015 Haiku, Inc. All rights reserved.
  * Distributed under the terms of the MIT License.
  *
  * Authors:
  *		Stefano Ceccherini, [email protected]
+ *		Julian Harnath, <[email protected]>
  */
 #ifndef _PICTURE_DATA_WRITER_H
 #define _PICTURE_DATA_WRITER_H
@@ -17,6 +18,7 @@
 #include <stack>
 
 
+class Layer;
 class BPositionIO;
 class BRegion;
 
@@ -91,6 +93,8 @@ public:
 			status_t			WriteDrawPicture(const BPoint& where,
 									const int32& token);
 
+			status_t			WriteBlendLayer(Layer* layer);
+
 protected:
 	// throw a status_t on error
 			void				BeginOp(const int16& op);

headers/private/interface/PictureProtocol.h

@@ -52,10 +52,11 @@ enum {
 	B_PIC_SET_FONT_BPP			= 0x0388,
 	B_PIC_SET_FONT_FACE			= 0x0389,
 	B_PIC_SET_TRANSFORM			= 0x0390,
+	B_PIC_BLEND_LAYER			= 0x0391
 };
 
 
-const static uint32 kOpsTableSize = 49;
+const static uint32 kOpsTableSize = 50;
 
 
 #endif