Add api for accessing backend texture via epi (#695)

* Define NativeTexture trait for offscreen rendering
add demo for NativeTexture trait

* write changelog

* add comment for native texture example

* formatting

* add license of Rust logo

* NativeTexture trait method rename
remove duplicate function with native texture
remove rust logo

* deprecated notice for register_glium_texture,register_webgl_texture

* collect deprecated notice

egui_glium/src/lib.rs

@@ -40,7 +40,8 @@ use {
     std::hash::{Hash, Hasher},
 };
 
-pub use copypasta::ClipboardContext; // TODO: remove
+pub use copypasta::ClipboardContext;
+use std::borrow::BorrowMut; // TODO: remove
 
 pub struct GliumInputState {
     pub pointer_pos_in_points: Option<Pos2>,
@@ -644,4 +645,8 @@ impl EguiGlium {
             &self.egui_ctx.texture(),
         );
     }
+
+    pub fn painter_mut(&mut self) -> &mut Painter {
+        self.painter.borrow_mut()
+    }
 }