The responsibilities of the Scene must be reduced to painting only so we
can move forward with the layer-based compositing.
This change moves direct scanout logic from the opengl scene to the base
scene class and the compositor. It makes the opengl scene less
overloaded and allows to share direct scanout logic.
This makes the Scene less overloaded and it's needed for things such as
render layers.
In hindsight, it would be great to merge checkGraphicsReset() and
beginFrame(), e.g. make beginFrame() return the status like in QRhi or
VkSwapchain. If it's OUT_OF_DATE or something, reinitialize the
compositor.
This unifies frame hooks for OpenGL and QPainter render backends. There
are a couple of reasons why it's a good idea - it provides one mental
framework to start painting a frame, the Compositor will be able to
start and submit frames. The last one is very cool because it gives the
Compositor more power over compositing.
Besides unifying frame hooks, this cleans up a bit the arg naming mess
in endFrame(). As is, "damage" and "damagedRegion" are very confusing
names. "damage" arg has been renamed to "renderedRegion," because that's
what it is. The renderedRegion arg specifies the region that has been
repainted by the Scene. It's different from the damagedRegion as that
one specifies the surface damage, i.e. the difference between the
current and the next frame, while the renderedRegion may include a
region that had to be repainted to repair the back buffer. The main
reason why we need renderedRegion is the X11 platform. On Wayland, it's
unused.
In the future, we will need to extend this api with output layers.
The main idea behind the render backend is to decouple low level bits
from scenes. The end goal is to make the render backend provide render
targets where the scene can render.
Design-wise, such a split is more flexible than the current state, for
example we could start experimenting with using qtquick (assuming that
the legacy scene is properly encapsulated) or creating multiple scenes,
for example for each output layer, etc.
So far, the RenderBackend class only contains one getter, more stuff will
be moved from the Scene as it makes sense.
