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_glow/src/post_process.rs

@@ -116,7 +116,7 @@ impl PostProcess {
         gl.bind_buffer(glow::ARRAY_BUFFER, Some(pos_buffer));
         gl.buffer_data_u8_slice(
             glow::ARRAY_BUFFER,
-            crate::misc_util::as_u8_slice(&positions),
+            bytemuck::cast_slice(&positions),
             glow::STATIC_DRAW,
         );