Support for HiDPI in OpenGL on Linux / GTK3.

wxGTK3 HiDPI support seems to emulate what OSX does quite closely,
thus the changes are relatively minimal.

Also fixed an ugly rounding issue when populating the ImGUI font map
with image thumbnails.

Fixes Gtk3 issue on 4k+ screens #4135
Fixes HiDPI screens with Wayland on Fedora 30 cause Plater view to be too small. #3245

src/slic3r/GUI/MainFrame.cpp

@@ -228,7 +228,8 @@ DPIFrame(NULL, wxID_ANY, "", wxDefaultPosition, wxDefaultSize, wxDEFAULT_FRAME_S
 // OSX specific issue:
 // When we move application between Retina and non-Retina displays, The legend on a canvas doesn't redraw
 // So, redraw explicitly canvas, when application is moved
-#if ENABLE_RETINA_GL
+//FIXME maybe this is useful for __WXGTK3__ as well?
+#if __APPLE__
     Bind(wxEVT_MOVE, [this](wxMoveEvent& event) {
         wxGetApp().plater()->get_current_canvas3D()->set_as_dirty();
         wxGetApp().plater()->get_current_canvas3D()->request_extra_frame();