Screenshot: Move filesystem I/O to a thread

Writting the screenshot to a file can take a relativly long time
in which we block the compositor, so do that part in a separate
thread to avoid the hang.

https://bugzilla.gnome.org/show_bug.cgi?id=652952

src/shell-global-private.h

@@ -30,6 +30,8 @@ typedef struct _screenshot_data {
   int width;
   int height;
 
+  cairo_surface_t *image;
+
   ShellGlobalScreenshotCallback callback;
 } _screenshot_data;