FontContext: Cache data fetched from the cache thread

Before this change, if we needed to create a Font which we've already
created, but at a new size, then we'd fetch the FontTemplateInfo again.
If the bytes of the font are held in memory, then this could be
expensive as we need to pass those bytes over IPC.

components/gfx/font_cache_thread.rs

@@ -32,7 +32,7 @@ pub struct FontTemplates {
     templates: Vec<FontTemplate>,
 }
 
-#[derive(Debug, Deserialize, Serialize)]
+#[derive(Clone, Debug, Deserialize, Serialize)]
 pub struct FontTemplateInfo {
     pub font_template: Arc<FontTemplateData>,
     pub font_key: webrender_api::FontKey,