clutter: Change stage view scale to be float

To support fractional scaling, change the stage view scale to be a
float instead of an int. Also change the places where it is retrieved
and used when scaling things.

https://bugzilla.gnome.org/show_bug.cgi?id=765011

src/backends/native/meta-renderer-native.c

@@ -1743,7 +1743,7 @@ meta_renderer_native_create_view (MetaRenderer       *renderer,
 
   view = g_object_new (META_TYPE_RENDERER_VIEW,
                        "layout", &logical_monitor->rect,
-                       "scale", scale,
+                       "scale", (float) scale,
                        "framebuffer", onscreen,
                        "offscreen", offscreen,
                        "logical-monitor", logical_monitor,