fix gl context memory leak on linux (#215)

Author: Quant1um

src/gl/x11.rs

@@ -241,5 +241,9 @@ impl GlContext {
 }
 
 impl Drop for GlContext {
-    fn drop(&mut self) {}
+    fn drop(&mut self) {
+        unsafe {
+            glx::glXDestroyContext(self.display, self.context);
+        }
+    }
 }

src/x11/window.rs

@@ -94,16 +94,17 @@ impl Drop for ParentHandle {
 }
 
 pub(crate) struct WindowInner {
+    // GlContext should be dropped **before** XcbConnection is dropped
+    #[cfg(feature = "opengl")]
+    gl_context: Option<GlContext>,
+
     pub(crate) xcb_connection: XcbConnection,
     window_id: XWindow,
     pub(crate) window_info: WindowInfo,
     visual_id: Visualid,
     mouse_cursor: Cell<MouseCursor>,
 
     pub(crate) close_requested: Cell<bool>,
-
-    #[cfg(feature = "opengl")]
-    gl_context: Option<GlContext>,
 }
 
 pub struct Window<'a> {