Variable font support (#400)

* Variable font support

Here's a pretty naïve solution for variable fonts.

The iterator doesn't use the match keys' weight, but instead tries to
get the requested ideal weight, if the font is variable, otherwise it is
ignored and the actual (non-variable) weight is used. This is because I
didn't implement finding variable weight support for match keys; doing
so would be impossible without parsing TTF files when matching and I
didn't want to add that potentially expensive infrastructure if not
entirely necessary.

This is a breaking change, and I'm open for ideas on how to fix that
if it's an issue.

* cargo fmt

* Add variable font example to rich-text example

src/layout.rs

@@ -16,6 +16,8 @@ pub struct LayoutGlyph {
     pub end: usize,
     /// Font size of the glyph
     pub font_size: f32,
+    /// Font weight of the glyph
+    pub font_weight: fontdb::Weight,
     /// Line height of the glyph, will override buffer setting
     pub line_height_opt: Option<f32>,
     /// Font id of the glyph
@@ -79,6 +81,7 @@ impl LayoutGlyph {
                 (self.x + x_offset) * scale + offset.0,
                 math::truncf((self.y - y_offset) * scale + offset.1), // Hinting in Y axis
             ),
+            self.font_weight,
             self.cache_key_flags,
         );