libwayland-egl: new virtual package

Currently, when wayland and EGL support are selected, mesa3d builds the
libwayland-egl library, which allows Wayland to use the libEGL
implementation provided by mesa3d.

But some embedded hardware devices like Raspberry Pi provides their own
implementation of libEGL. So it is likely that an implementation of
libwayland-egl for RPi will appear sooner or later.

So a new virtual package called libwayland-egl is added, to handle these
future cases.
mesa3d: provides libwayland-egl

Declare mesa3d as a provider for libwayland-egl, when wayland is
selected and EGL support is enabled.
cogl: new package

Cogl is an open source GPU graphics and utilities API providing:

- a hardware accelerated graphics API.
- math utilities (matrices, vectors, ...).
- texturing utilities.
clutter: new package.

Clutter is an open source software library for creating fast,
compelling, portable, and dynamic graphical user interfaces.

It depends on cogl and has support for X11, Wayland.