gtk3: die on SIGINT (Ctrl-C)

https://bugzilla.gnome.org/show_bug.cgi?id=622084
brushlib: fix a examples and testing somewhat

Fix a couple of out-of-bound access mainly in fixed-tiled-surface.c.
Fix segfault due to missing file. The example is still completely
broken, but at least nothing seems to segfault any more.
Merge branch 'mypaint-head'