Fix color blending in fontdue renderer

Dividing by 256 (instead of correct 255) produced GIFs that were 0.4%
darker than they should have been.

src/renderer/fontdue.rs

@@ -159,9 +159,9 @@ fn mix_colors(fg: RGBA8, bg: RGBA8, ratio: u8) -> RGBA8 {
     let ratio = ratio as u16;
 
     RGBA8::new(
-        ((bg.r as u16) * (255 - ratio) / 256) as u8 + ((fg.r as u16) * ratio / 256) as u8,
-        ((bg.g as u16) * (255 - ratio) / 256) as u8 + ((fg.g as u16) * ratio / 256) as u8,
-        ((bg.b as u16) * (255 - ratio) / 256) as u8 + ((fg.b as u16) * ratio / 256) as u8,
+        ((bg.r as u16) * (255 - ratio) / 255) as u8 + ((fg.r as u16) * ratio / 255) as u8,
+        ((bg.g as u16) * (255 - ratio) / 255) as u8 + ((fg.g as u16) * ratio / 255) as u8,
+        ((bg.b as u16) * (255 - ratio) / 255) as u8 + ((fg.b as u16) * ratio / 255) as u8,
         255,
     )
 }