Virtual monitors are monitors that isn't backed by any monitor like
hardware. It would typically be backed by e.g. a remote desktop service,
or a network display.
It is currently only supported by the native backend, and whether the
X11 backend will ever see virtual monitors is an open question. This
rest of this commit message describes how it works under the native
backend.
Each virutal monitor consists of virtualized mode setting components:
* A virtual CRTC mode (MetaCrtcModeVirtual)
* A virtual CRTC (MetaCrtcVirtual)
* A virtual connector (MetaOutputVirtual)
In difference to the corresponding mode setting objects that represents
KMS objects, the virtual ones isn't directly tied to a MetaGpu, other
than the CoglFramebuffer being part of the GPU context of the primary
GPU, which is the case for all monitors no matter what GPU they are
connected to. Part of the reason for this is that a MetaGpu in practice
represents a mode setting device, and its CRTCs and outputs, are all
backed by real mode setting objects, while a virtual monitor is only
backed by a framebuffer that is tied to the primary GPU. Maybe this will
be reevaluated in the future, but since a virtual monitor is not tied to
any GPU currently, so is the case for the virtual mode setting objects.
The native rendering backend, including the cursor renderer, is adapted
to handle the situation where a CRTC does not have a GPU associated with
it; this in practice means that it e.g. will not try to upload HW cursor
buffers when the cursor is only on a virtual monitor. The same applies
to the native renderer, which is made to avoid creating
MetaOnscreenNative for views that are backed by virtual CRTCs, as well
as to avoid trying to mode set on such views.
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1698>
