Fix issues with Atlas leaking memory.

* Fix issue with Font Atlas leaking memory by adding a drop impl, reported in issue #627

src/text/atlas.rs

@@ -1,5 +1,5 @@
 use crate::{
-    get_quad_context,
+    get_context, get_quad_context,
     math::Rect,
     texture::{Image, Texture2D},
     Color,
@@ -32,6 +32,12 @@ pub struct Atlas {
     unique_id: u64,
 }
 
+impl Drop for Atlas {
+    fn drop(&mut self) {
+        self.delete();
+    }
+}
+
 impl Atlas {
     // pixel gap between glyphs in the atlas
     const GAP: u16 = 2;
@@ -184,4 +190,8 @@ impl Atlas {
             );
         }
     }
+    pub fn delete(&mut self) {
+        let ctx = &mut get_context().quad_context;
+        ctx.delete_texture(self.texture);
+    }
 }