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>

crates/epaint/src/text/fonts.rs

@@ -254,6 +254,50 @@ pub struct FontDefinitions {
     pub families: BTreeMap<FontFamily, Vec<String>>,
 }
 
+#[derive(Debug, Clone)]
+pub struct FontInsert {
+    /// Font name
+    pub name: String,
+
+    /// A `.ttf` or `.otf` file and a font face index.
+    pub data: FontData,
+
+    /// Sets the font family and priority
+    pub families: Vec<InsertFontFamily>,
+}
+
+#[derive(Debug, Clone)]
+pub struct InsertFontFamily {
+    /// Font family
+    pub family: FontFamily,
+
+    /// Fallback or Primary font
+    pub priority: FontPriority,
+}
+
+#[derive(Debug, Clone)]
+pub enum FontPriority {
+    /// Prefer this font before all existing ones.
+    ///
+    /// If a desired glyph exists in this font, it will be used.
+    Highest,
+
+    /// Use this font as a fallback, after all existing ones.
+    ///
+    /// This font will only be used if the glyph is not found in any of the previously installed fonts.
+    Lowest,
+}
+
+impl FontInsert {
+    pub fn new(name: &str, data: FontData, families: Vec<InsertFontFamily>) -> Self {
+        Self {
+            name: name.to_owned(),
+            data,
+            families,
+        }
+    }
+}
+
 impl Default for FontDefinitions {
     /// Specifies the default fonts if the feature `default_fonts` is enabled,
     /// otherwise this is the same as [`Self::empty`].

crates/epaint/src/text/mod.rs

@@ -10,7 +10,10 @@ mod text_layout_types;
 pub const TAB_SIZE: usize = 4;
 
 pub use {
-    fonts::{FontData, FontDefinitions, FontFamily, FontId, FontTweak, Fonts, FontsImpl},
+    fonts::{
+        FontData, FontDefinitions, FontFamily, FontId, FontInsert, FontPriority, FontTweak, Fonts,
+        FontsImpl, InsertFontFamily,
+    },
     text_layout::layout,
     text_layout_types::*,
 };