hidgl: Provide more explicit management of rendering instances

hidgl: Provide more explicit management of rendering instances

Provide an (hidgl_instance *) pointer which is used to refer to a hidgl
instance when caling hidgl_* APIs that are instance, but not hidGC specific.

Various hidgl APIs now take the hidgl_instance as a parameter, or in the case
of hidgl_new_instance(), return a new instance.

Graphics APIs which previously had a hidGC parameter remain the same, as the
hidglGC structure now contains a link to the hidgl instance which created it.

This change paves the way to potentially having multiple hidgl instances active
simultaneously, and removes various global variables (such as "buffer" and
"global_depth") which were previously accessible to allow the gtk-gl HID to
draw using the hidgl internals.

The gtk-gl HID_DRAW graphics API now subclasses the hidgl HID_DRAW API, so the
gtk-gl HID can access the (non-private) gtk_gc->hidgl_gc.* fields.