This commit brings a big change to the DND and copy/paste interfaces.
Most importantly the functionality is now independent of wl_shell.
The wl_shell interface is intended for desktop style UI interaction and
an optional and experimental interface.
The new interface also allows receiving the DND data multiple times or
multiple times during the drag, and the mechanism for offering and receiving
data is now shared between DND and selections.
Some events, such as the display.delete_id, aren't very urgent and we
would like to not always send them immdiately and cause an unnecessary
context switch. The wl_resource_queue_event() function will place the
event in the connection output buffer but not request the main loop to
poll for writable. The effect is that the event will just sit in the
output buffer until a more important event comes around and requires
flushing.
The visual interface was meant to be a generic mechanism for
specifying the content of a buffer. It goes back to before we had the
buffer factory interfaces (like wl_drm and wl_shm) and we wanted to
keep it open-ended enough that yuv, png or even svg buffer or so would
be possible.
Now that we have the buffer abstraction, we can add different buffer
types by introducing new interfaces that create buffers. It only
makes sense to leave it to those interfaces to specify the contents of
the buffers.
For wl_shm, this means that we now just specify the pixel format using
an enum. For EGL buffers, the exact pixel formats are controlled by
the implementation (part of wl_drm and similar), and from the client
point of view, everything is controlled using EGLConfigs.
