Enable Retina (HiDPI) support on OSX wxGLCanvas

Closes #15700.

Closes https://github.com/wxWidgets/wxWidgets/pull/1485

src/osx/cocoa/glcanvas.mm

@@ -167,6 +167,7 @@ bool wxGLCanvas::DoCreate(wxWindow *parent,
     
     NSRect r = wxOSXGetFrameForControl( this, pos , size ) ;
     wxNSCustomOpenGLView* v = [[wxNSCustomOpenGLView alloc] initWithFrame:r];
+    [v setWantsBestResolutionOpenGLSurface:YES];
     
     wxWidgetCocoaImpl* c = new wxWidgetCocoaImpl( this, v, wxWidgetImpl::Widget_UserKeyEvents | wxWidgetImpl::Widget_UserMouseEvents );
     SetPeer(c);