draw: call vfunc rather then emit signal for the common case
draw: call vfunc rather then emit signal for the common case

This avoids a lot of overhead in the common case where a signal
is not connected and we're just using the class vfunc (which is true
for all in-libgtk widgets). Additionally it makes backtraces in
debuggers and profiles much much nicer to look at.

https://bugzilla.gnome.org/show_bug.cgi?id=754986

davidgumberg/gtk,jadahl/gtk,jigpu/gtk,alexlarsson/gtk,davidgumberg/gtk,jadahl/gtk,jigpu/gtk,jadahl/gtk,jigpu/gtk,jadahl/gtk,jigpu/gtk,grubersjoe/adwaita,davidgumberg/gtk,alexlarsson/gtk,davidgumberg/gtk,grubersjoe/adwaita,grubersjoe/adwaita,grubersjoe/adwaita,jadahl/gtk,jigpu/gtk,jigpu/gtk,alexlarsson/gtk,grubersjoe/adwaita,jadahl/gtk,davidgumberg/gtk,davidgumberg/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,alexlarsson/gtk,davidgumberg/gtk,grubersjoe/adwaita,alexlarsson/gtk,alexlarsson/gtk,jadahl/gtk,grubersjoe/adwaita,jigpu/gtk,alexlarsson/gtk,davidgumberg/gtk