Switch present mode to "mailbox"

I'm running into this issue:
gfx-rs/wgpu#1218

I'm not aware of any adverse effects of switching to
`PresentMode::Mailbox`, except that it's "not optimal for mobile",
according to the wgpu documentation. Since we don't currently support
any mobile platforms, I think this is fine for now.

fj-app/src/graphics/renderer.rs

@@ -75,7 +75,7 @@ impl Renderer {
             format: color_format,
             width: window.width(),
             height: window.height(),
-            present_mode: wgpu::PresentMode::Fifo,
+            present_mode: wgpu::PresentMode::Mailbox,
         };
         surface.configure(&device, &surface_config);