drawinglayer: Improve the README.

Change-Id: I40ae115a03f1a289c940f9f96e3d496d1eba1a48

drawinglayer/README

-Somewhat of a middle layer between rendering subsystems and application cores.
+Drawing API that can specify what to draw via a kind of display list.
 
-(What are "rendering subsystems" and "application cores"?)
+Example of the DrawingLayer use is eg. in svx/source/xoutdev/xtabhtch.cxx:121.
+A stripped down version with extended comments:
 
-It offers a nice and easy API to render complex shapes. (Pure "marketing".)
+     // Create a hatch primitive (here a rectangle that will be filled with
+     // the appropriate hatching, but has no border).
+     // This will not draw it yet; it's so far only constructed to add it to a
+     // display list later.
+     const drawinglayer::primitive2d::Primitive2DReference aHatchPrimitive(
+         new drawinglayer::primitive2d::PolyPolygonHatchPrimitive2D(...));
+
+     // Create a rectangle around the hatch, to give it a border.
+     const drawinglayer::primitive2d::Primitive2DReference aBlackRectanglePrimitive(
+         new drawinglayer::primitive2d::PolygonHairlinePrimitive2D(...));
+
+     // Here we want to render to a virtual device (to later obtain the bitmap
+     // out of that), so prepare it.
+     VirtualDevice aVirtualDevice;
+
+     // Create processor and draw primitives, to get it ready for rendering.
+     boost::scoped_ptr<drawinglayer::processor2d::BaseProcessor2D> pProcessor2D(
+         drawinglayer::processor2d::createPixelProcessor2DFromOutputDevice(...));
+
+     if (pProcessor2D)
+     {
+         // Fill-in the display list.
+         drawinglayer::primitive2d::Primitive2DSequence aSequence(2);
+
+         aSequence[0] = aHatchPrimitive;
+         aSequence[1] = aBlackRectanglePrimitive;
+
+         // Render it to the virtual device.
+         pProcessor2D->process(aSequence);
+         pProcessor2D.reset();
+     }
+
+     // Obtain the bitmap that was rendered from the virtual device, to re-use
+     // it in the widget.
+     aRetval = aVirtualDevice.GetBitmap(Point(0, 0), aVirtualDevice.GetOutputSizePixel());
+
+== DrawingLayer glossary ==
+
+Primitives - classes that represent what should be drawn.  It holds the data
+what to draw, but does not contain any kind of the rendering.  Some of the
+primitives are 'Basic primitives', that are primitives that cannot be
+decomposed.  The rest of the primitives can be decomposed to the basic
+primitives.
+
+Decomposition - a way how to break down the more complicated primitives into
+the basic primitives, and represent them via them; this logically makes the
+plain Primitive2DSequence display list a hierarchy.
+Eg. PolygonMarkerPrimitive2D can be decomposed to 2 hairlines
+PolyPolygonHairlinePrimitive2D's, each with different color.
+
+Processor - a class that goes through the hierarchy of the Primitives, and
+renders it some way.  Various processors, like VclPixelProcessor2D (renders to
+the screen), VclMetafileProcessor2D (renders to the VCL metafile, eg. for
+printing), etc.
+
+== How to Implement a new Primitive ("something new to draw") ==
+
+* Create an ancestor of BasePrimitive2D
+  (or of its ancestor if it fits the purpose better)
+
+  * Assign it an ID [in drawinglayer_primitivetypes2d.hxx]
+
+  * Implement its decomposition
+    [virtual Primitive2DSequence create2DDecomposition(...)]
+
+* Extend the (various) processor(s)
+  If you need more than relying on just the decomposition
+
+== Where is DrawingLayer used ==
+
+* SdrObject(s) (rectangles, Circles, predefined shapes etc.)
+
+* Selections
+
+* Various smaller cases to 'just draw something'
+
+  * Draw to a virtual device, and use the resulting bitmap (like the example
+    above)
+
+* Custom widgets (like the Header / Footer indicator button)