Fix disappearing text in vertical layouts with word-wrap and alignment

When a Text element has wrapping enabled, it should not have zero
minimum size. Otherwise the layout will give it a height of zero in the
layout in the test case and that'll make the text disappear. There are
different options but this patch goes for minimum height as if no
wrapping was enabled (so at least one line plus forced line breaks).

Fixes #246

sixtyfps_runtime/rendering_backends/testing/lib.rs

@@ -159,6 +159,10 @@ impl FontMetrics for TestingFontMetrics {
         Size::new(text.len() as f32 * 10., 10.)
     }
 
+    fn line_height(&self) -> f32 {
+        10.
+    }
+
     fn text_offset_for_x_position(&self, _text: &str, _x: f32) -> usize {
         0
     }