text: font: add owner field to font_ops

text: font: add owner field to font_ops

The owner fields specifies the module that provides the code for the given
backend. The font-core is responsible of ref/unref'ing the module so its
code is always available as long as the module exists.

We cannot push this into the modules as modules _must_ never call
kmscon_module_unref(KMSCON_THIS_MODULE)! Because this might drop the last
reference and hence the function might disallocate the module. However,
this means that it cannot return because the caller is _part_ of the
module and so no longer valid.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>