Make canvas_text example render on expose

Before this change, the `canvas_text` example would become blank after
navigating to another workspace or being put beneath another window.
This change will listen for the SDL window exposed event, and will
render then instead of only at the beginning.

Author: leroycep

examples/canvas_text/src/main.rs

@@ -20,7 +20,7 @@ use pathfinder_renderer::gpu::renderer::Renderer;
 use pathfinder_renderer::options::BuildOptions;
 use pathfinder_resources::ResourceLoader;
 use pathfinder_resources::fs::FilesystemResourceLoader;
-use sdl2::event::Event;
+use sdl2::event::{Event, WindowEvent};
 use sdl2::keyboard::Keycode;
 use sdl2::video::GLProfile;
 use std::iter;
@@ -64,28 +64,29 @@ fn main() {
     let font = Handle::from_memory(font_data, 0);
     let font_context = CanvasFontContext::from_fonts(iter::once(font));
 
-    // Make a canvas.
-    let mut canvas = Canvas::new(window_size.to_f32()).get_context_2d(font_context);
-
-    // Draw the text.
-    canvas.set_font("Overpass-Regular");
-    canvas.set_font_size(32.0);
-    canvas.fill_text("Hello Pathfinder!", vec2f(32.0, 48.0));
-    canvas.set_text_align(TextAlign::Right);
-    canvas.stroke_text("Goodbye Pathfinder!", vec2f(608.0, 464.0));
-
-    // Render the canvas to screen.
-    let mut scene = SceneProxy::from_scene(canvas.into_canvas().into_scene(),
-                                           renderer.mode().level,
-                                           RayonExecutor);
-    scene.build_and_render(&mut renderer, BuildOptions::default());
-    window.gl_swap_window();
-
     // Wait for a keypress.
     let mut event_pump = sdl_context.event_pump().unwrap();
     loop {
         match event_pump.wait_event() {
             Event::Quit {..} | Event::KeyDown { keycode: Some(Keycode::Escape), .. } => return,
+            Event::Window { win_event: WindowEvent::Exposed, .. } => {
+                // Make a canvas.
+                let mut canvas = Canvas::new(window_size.to_f32()).get_context_2d(font_context.clone());
+
+                // Draw the text.
+                canvas.set_font("Overpass-Regular");
+                canvas.set_font_size(32.0);
+                canvas.fill_text("Hello Pathfinder!", vec2f(32.0, 48.0));
+                canvas.set_text_align(TextAlign::Right);
+                canvas.stroke_text("Goodbye Pathfinder!", vec2f(608.0, 464.0));
+
+                // Render the canvas to screen.
+                let mut scene = SceneProxy::from_scene(canvas.into_canvas().into_scene(),
+                                                       renderer.mode().level,
+                                                       RayonExecutor);
+                scene.build_and_render(&mut renderer, BuildOptions::default());
+                window.gl_swap_window();
+            },
             _ => {}
         }
     }