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

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.

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