theme: draw hover overlay in the correct scale

theme: draw hover overlay in the correct scale

The hover icon buffer on which the hover overlay is draw can be larger
than the button size when a large non-hover icon is provided by user. In
this case, we should scale up all the coorinates that are used to draw
the hover overlay. Before this commit, corner radius and padding.width
are not scaled correctly.

This commit fixes it by scaling the cairo context with cairo_scale()
when drawing the hover overlay.