annotations: widen the user bounding box by half a pixel on all sides (…

…#305)

When drawing anchored annotations, widen the user bounding box since the pixels are centered on the integers of the user coordinates.

Fixes #223.

src/annotations.jl

@@ -296,9 +296,11 @@ function Gtk4.draw(c::Gtk4.GtkCanvas, ann::AnchoredAnnotation)
         ctx = getgc(c)
         Graphics.save(ctx)
         data = ann.data
-        set_coordinates(ctx, ann.devicebb(data), ann.userbb(data))
-        scale_x = width(ann.userbb(data))/width(ann.devicebb(data))
-        scale_y = height(ann.userbb(data))/height(ann.devicebb(data))
+        userbb = BoundingBox(ann.userbb(data).xmin-0.5,ann.userbb(data).xmax+0.5,ann.userbb(data).ymin-0.5,ann.userbb(data).ymax+0.5)
+        devicebb = ann.devicebb(data)
+        set_coordinates(ctx, ann.devicebb(data), userbb)
+        scale_x = width(userbb)/width(devicebb)
+        scale_y = height(userbb)/height(devicebb)
         draw_anchored(ctx, data, scale_x, scale_y)
         restore(ctx)
     end