While porting GTK to GskRenderer we noticed that the current fallback
code for widgets using Cairo to draw is not enough to cover all the
possible cases.
For instance, if a container widget still uses GtkWidget::draw to render
its children, and at least one of them has been ported to using render
nodes instead, the container won't know how to draw it.
For this reason we want to provide to layers above GSK the ability to
create a "fallback" renderer instance, created using a "parent"
GskRenderer instance, but using a Cairo context as the rendering target
instead of a GdkDrawingContext.
GTK will use this inside the gtk_widget_draw() implementation, if a
widget implements GtkWidgetClass.get_render_node().
We're going to need to allow rendering on a specific cairo_t in order to
implement fallback code paths inside GTK; this means that there will be
times when we have a transient GskRenderer instance that does not have a
GdkDrawingContext to draw on.
Instead of adding a new render() implementation for those cases and then
decide which one to use, we can remove the drawing context argument from
the virtual function itself, and allow using a NULL GdkDrawingContext
when calling gsk_renderer_render(). A later commit will add a generic
function to create a transient GskRenderer with a cairo_t attached to
it.
Renderers inside GSK will have to check whether we have access to a
GdkDrawingContext, in which case we're going to use it; or if we have
access to a cairo_t and a window.
This commit changes the way GskRenderer and GskRenderNode interact and
are meant to be used.
GskRenderNode should represent a transient tree of rendering nodes,
which are submitted to the GskRenderer at render time; this allows the
renderer to take ownership of the render tree. Once the toolkit and
application code have finished assembling it, the render tree ownership
is transferred to the renderer.
GSK is conceptually split into two scene graphs:
* a simple rendering tree of operations
* a complex set of logical layers
The latter is built on the former, and adds convenience and high level
API for application developers.
The lower layer, though, is what gets transformed into the rendering
pipeline, as it's simple and thus can be transformed into appropriate
rendering commands with minimal state changes.
The lower layer is also suitable for reuse from more complex higher
layers, like the CSS machinery in GTK, without necessarily port those
layers to the GSK high level API.
This lower layer is based on GskRenderNode instances, which represent
the tree of rendering operations; and a GskRenderer instance, which
takes the render nodes and submits them (after potentially reordering
and transforming them to a more appropriate representation) to the
underlying graphic system.
