Enable SRGB (#1178)

This will solve the error "Error creating GL context; Couldn't find any
pixel format that matches the criterias."

(I think this maybe a problem of mesa (18.0.0_rc4), but I'm not sure.

See the thread <https://mastodon.cardina1.red/@lo48576/99670278063669603>
for my debug log.

src/window.rs

@@ -200,6 +200,7 @@ impl Window {
             .with_transparency(true)
             .with_decorations(window_config.decorations());
         let context = ContextBuilder::new()
+            .with_srgb(true)
             .with_vsync(true);
         let window = ::glutin::GlWindow::new(window, context, &event_loop)?;
         window.show();