As we don't have GLPLatform before the backend is fully created
the AbstractEglBackend has a new method isOpenGLES() -> bool
which determines based on QOpenGLContext::openGLModuleType().
This change introduces a mechanism for internal windows to be rendered
to a QOpenGLFramebufferObject to be composited using the texture bound
to the FBO. This is useful for in-process rendering (e.g. QtQuick) and
at the same time bypassing the windowing system.
The OpenGL context of the QOpenGLFramebufferObject needs to be sharing
with the compositing OpenGL context.
Each of the backends becomes a plugin. This allows kwin_wayland to load
the requested plugin and kwin itself doesn't need to link all the
libraries needed. E.g. libdrm is no longer linked if running kwin_x11.
Also this allows to create backends for the non-standard EGL platforms
(examples could be raspberrypi or Android devices).
For each DrmOutput a gbm_surface and EglSurface is created. When
rendering per screen the context is made current on each of the
surfaces.
Note: viewport handling needs to be improved by e.g. passing through
the scene to restore to correct viewport after dropping an FBO.
Furthermore it seems like buffer age is not working correctly in this
setup (not overly surprising).
Dynamic changes are not yet supported.
There's quite some overlap and duplicated code. This AbstractEglBackend
tries to merge the two backends a little bit again.
This also introduces an AbstractEglTexture which supports both X11 and
Wayland "pixmaps" so that EglOnXBackend can support Wayland buffers.
