widget: Fix a drawing artifact with blinking cursor
widget: Fix a drawing artifact with blinking cursor

https://bugzilla.gnome.org/show_bug.cgi?id=731427
(cherry picked from commit 977375b05df4f906329177a401a2d8c00b2516d4)

pjm0616/vte,pjm0616/vte,pjm0616/vte