Context::add_font (#5228)

make it easier to add fonts. 

For example if I want to add a custom FontFamily or if the user wants to
add a Chinese fallback
* [x] I have followed the instructions in the PR template

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

examples/custom_font/src/main.rs

@@ -1,7 +1,10 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
 #![allow(rustdoc::missing_crate_level_docs)] // it's an example
 
-use eframe::egui;
+use eframe::{
+    egui,
+    epaint::text::{FontInsert, InsertFontFamily},
+};
 
 fn main() -> eframe::Result {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
@@ -16,7 +19,28 @@ fn main() -> eframe::Result {
     )
 }
 
-fn setup_custom_fonts(ctx: &egui::Context) {
+// Demonstrates how to add a font to the existing ones
+fn add_font(ctx: &egui::Context) {
+    ctx.add_font(FontInsert::new(
+        "my_font",
+        egui::FontData::from_static(include_bytes!(
+            "../../../crates/epaint_default_fonts/fonts/Hack-Regular.ttf"
+        )),
+        vec![
+            InsertFontFamily {
+                family: egui::FontFamily::Proportional,
+                priority: egui::epaint::text::FontPriority::Highest,
+            },
+            InsertFontFamily {
+                family: egui::FontFamily::Monospace,
+                priority: egui::epaint::text::FontPriority::Lowest,
+            },
+        ],
+    ));
+}
+
+// Demonstrates how to replace all fonts.
+fn replace_fonts(ctx: &egui::Context) {
     // Start with the default fonts (we will be adding to them rather than replacing them).
     let mut fonts = egui::FontDefinitions::default();
 
@@ -53,7 +77,8 @@ struct MyApp {
 
 impl MyApp {
     fn new(cc: &eframe::CreationContext<'_>) -> Self {
-        setup_custom_fonts(&cc.egui_ctx);
+        replace_fonts(&cc.egui_ctx);
+        add_font(&cc.egui_ctx);
         Self {
             text: "Edit this text field if you want".to_owned(),
         }