Only the X based Scenes need an overlay window, so the Compositor doesn't
need to check for it in the Wayland case.
OverlayWindow is moved from OpenGLBackend to the sub classes which need
to provide it.
Allow prepareRenderingFrame() to return a region that will be
repainted in addition to the damaged region.
Pass both the damaged region and the repainted region, which
may be larger, as parameters to endRenderingFrame().
With QtQuick2 it's possible that the scene graph rendering context either
lives in an own thread or uses the main GUI thread. In the latter case
it's the same thread as our compositing OpenGL context lives in. This
means our basic assumption that between two rendering passes the context
stays current does not hold.
The code already ensured that before we start a rendering pass the
context is made current, but there are many more possible cases. If we
use OpenGL in areas not triggered by the rendering loop but in response
to other events the context needs to be made current. This includes the
loading and unloading of effects (some effects use OpenGL in the static
effect check, in the ctor and dtor), background loading of texture data,
lazy loading after first usage invoked by shortcut, etc. etc.
To properly handle these cases new methods are added to EffectsHandler
to make the compositing OpenGL context current. These calls delegate down
into the scene. On non-OpenGL scenes they are noop, but on OpenGL they go
into the backend and make the context current. In addition they ensure
that Qt doesn't think that it's QOpenGLContext is current by calling
doneCurrent() on the QOpenGLContext::currentContext(). This unfortunately
causes an additional call to makeCurrent with a null context, but there
is no other way to tell Qt - it doesn't notice when a different context
is made current with low level API calls. In the multi-threaded
architecture this doesn't matter as ::currentContext() returns null.
A short evaluation showed that a transition to QOpenGLContext doesn't
seem feasible. Qt only supports either GLX or EGL while KWin supports
both and when entering the transition phase for Wayland, it would become
extremely tricky if our native platform is X11, but we want a Wayland
EGL context. A future solution might be to have a "KWin-QPA plugin" which
uses either xcb or Wayland and hides everything from Qt.
The API documentation is extended to describe when the effects-framework
ensures that an OpenGL context is current. The effects are changed to
make the context current in cases where it's not guaranteed. This has
been done by looking for creation or deletion of GLTextures and Shaders.
If there are other OpenGL usages outside the rendering loop, ctor/dtor
this needs to be changed, too.
The handling for creating and managing the OpenGL context is
split out of the SceneOpenGL into the abstract OpenGLBackend
and it's two subclasses GlxBackend and EglOnXBackend.
The backends take care of creating the OpenGL context on the
windowing system, e.g. on glx an OpenGL context on the overlay
window is created and in the egl case an EGL context is created.
This means that the SceneOpenGL itself does not have to care
about the specific underlying infrastructure.
Furthermore the backend provides the Textures for the specific
texture from pixmap operations. For that in each of the backend
files an additional subclass of the TexturePrivate is defined.
These subclasses hold the EglImage and GLXPixmap respectively.
The backend is able to create such a private texture and for
that the ctor of the Texture is changed to take the backend as
a parameter and the Scene provides a factory method for
creating Textures. To make this work inside Window the Textures
are now hold as pointers which seems a better choice anyway as
to the member functions pointers are passed.
