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.
According to spec eglQueryWaylandBufferWL returns EGL_FALSE if
EGL_WAYLAND_Y_INVERTED_WL is not supported and in that case we should
treat it as af the value were set to EGL_TRUE.
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.
