Fix underscores not displayed on every line (#316)

With certain font-configurations, underscores are not drawn on every
line.

Locally, setting `set guifont=Deja\ for\ Powerline\ Medium:h11` caused
underscores to be hidden on two lines and then displayed on the
following three.

Fixing the line height to the nearest integer seems to alleviate the
issue.

src/renderer/mod.rs

@@ -51,7 +51,7 @@ impl Renderer {
         if updated {
             let (font_width, font_height) = self.shaper.font_base_dimensions();
             self.font_width = font_width;
-            self.font_height = font_height;
+            self.font_height = font_height.ceil();
         }
         updated
     }