Fix error

I found the following error:
workarea.cpp:2737:17: error: no matching function for call to studio::WorkArea::refresh(GdkEventExpose*)'

So I did the change to simply refresh the workarea that is throw the queue_draw event.

But to reach this compiling error I needed to fix two lines of a library header according to this recipe:
http://inkscape.13.x6.nabble.com/inkscape-0-91pre2-source-tarball-td4971187.html
So finally I could completely build Synfig Studio on gtk branch on Windows.