imgui: bump copy

imgui: bump copy

Updated at :

commit f977871854af941289f2a9090dcc90f7aa3449a8
Author: omar <omarcornut@gmail.com>
Date:   Fri Feb 15 13:10:22 2019 +0100

    ImFont: Minor adjustment to the structure.
    Examples: Removed unused variable.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
+1-by: Mike Lothian <mike@fireburn.co.uk>
+1-by: Tapani Pälli <tapani.palli@intel.com>
+1-by: Eric Engestrom <eric.engestrom@intel.com>
+1-by: Yurii Kolesnykov <root@yurikoles.com>
+1-by: myfreeweb <greg@unrelenting.technology>
+1-by: Kenneth Graunke <kenneth@whitecape.org>