Texture loading in egui (#1110)

* Move texture allocation into epaint/egui proper
* Add TextureHandle
* egui_glow: cast using bytemuck instead of unsafe code
* Optimize glium painter
* Optimize WebGL
* Add example of loading an image from file

egui_demo_lib/src/syntax_highlighting.rs

@@ -66,7 +66,7 @@ enum SyntectTheme {
 
 #[cfg(feature = "syntect")]
 impl SyntectTheme {
-    fn all() -> impl Iterator<Item = Self> {
+    fn all() -> impl ExactSizeIterator<Item = Self> {
         [
             Self::Base16EightiesDark,
             Self::Base16MochaDark,