Ensure that absolute LineHeight is always > 0.0

Fixes issue #2002.
#2002

The gist of the problem is that `LineHeight::to_absolute` in conjunction
with other widgets may inadvertently return `0.0`. This triggers a panic in `cosmic_text`.

The original issue has a minimal example.

core/src/text.rs

@@ -74,7 +74,9 @@ impl LineHeight {
     /// Returns the [`LineHeight`] in absolute logical pixels.
     pub fn to_absolute(self, text_size: Pixels) -> Pixels {
         match self {
-            Self::Relative(factor) => Pixels(factor * text_size.0),
+            Self::Relative(factor) => {
+                Pixels((factor * text_size.0).max(f32::MIN_POSITIVE))
+            }
             Self::Absolute(pixels) => pixels,
         }
     }