mutter plans for 3.22

[Matthias Clasen <matthias clasen gmail com>]

More feature planning for 3.22:

We've put a lot of effort into getting Wayland to feature parity, and
almost got there. Some of the remaining loose ends require closer
integration with cogl and clutter,
for example:
- monitor rotation
- hotplugging graphics devices
- output on secondary gpus
- fractional scaling
- injecting input events (e.g. for remoting)

To make this possible, we (myself, Jonas Adahl, Rui Matos, Carlos
Garnacho and Ray Strode) come up with a plan that roughly looks as
follows:

1) put clutter and cogl in mutter
2) remove irrelevant backends from clutter and cogl (leave X11 and KMS)
3) move the clutter cogl, native and x11 backend into mutter
4) move cogls kms and x11 winsys backends into mutter (i.e. Meta*... classes)
5) make it possible for clutter/cogl to draw to multiple onscreen framebuffers
6) make clutter render each (rectangular) region with the same scale indivudally
    onto a particular onscreen framebuffer.

Some existing work along these lines is being tracked in
https://bugzilla.gnome.org/show_bug.cgi?id=760439

As part of these code reorganization, we should also look at
separating the gbm code in cogl, with the goal of adding support for
nvidias apis next to it.