SystemUI: Reallocate font texture when screen size changes.
SystemUI: Reallocate font texture when screen size changes.

Fixes #60 (font becoming corrupted after window is resized and editor is restarted).

rokups/Urho3D,rokups/Urho3D,rokups/Urho3D,rokups/Urho3D,rokups/Urho3D,rokups/Urho3D