This is simple since no deferred release queue is used here.

Sharable resources, which are at least buffers and textures, can now be
orphaned, meaning the owning QRhi can be destroyed before releasing the
QRhiResource. They will still function in a limited way (cannot be rebuilt
anymore) as long as there is at least one QRhi associated with the
QRhiResourceSharingHost (i.e. as long as the share group is non-empty).

...in order to reduce code duplication.

Does not really belong there anyway

The threaded case cannot be supported on all backends for now.
Revert parts of the previous d3d patch.

Some day we may consider exploring the possibility of doing something
with deferred contexts on d3d11 but that's not an option for now.

Won't yet work, obviously.

...and allow importing an already existing QOpenGLContext via
QRhiGles2NativeHandles. This way the API is fully consistent with other
backends and typical applications become shorter due to not having to
create a context, having a convenience function to create the
QOffscreenSurface, and not having to worry about depth/stencil in
QSurfaceFormat.

Eliminate most of the structs, inner structs, and inline ctors

Here we choose to go the heavy weight route and turn it into a
d-pointered (implicitly shared, even) class since the data a binding
description needs may grow in yet-unknown ways when introducing support
for other resource types.

...and some leftover docs

...and mark the first round of doc work as done, yay