Always add a mouse driver instance configured to send core events, unless
a core pointer already exists using either the mouse or void drivers. This
handles the laptop case where the config file only specifies, say,
synaptics, which causes the touchpad to work but not the pointing stick.
We don't double-instantiate the mouse driver to avoid the mouse moving twice
as fast, and we skip this logic when the user asked for a void core pointer
since that probably means they want to run with no pointer at all.
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