shared/cairo-util: Release any fontmap laying around

This is particularly useful when using the weston-editor which seems to
cause an issue with cairo_debug_reset_static_data(), as that still seems
to find out there are references laying around, causing a crash when
exiting:

 ../../../../src/cairo-hash.c:217: _cairo_hash_table_destroy: Assertion
 `hash_table->live_entries == 0' failed

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>