Add rectangle toggle example

This commit adds a new example that shows a XOR pattern. A rectangle can
be superimposed on the background by pressing the space bar.

This example was created to demonstrate a bug in the Core Graphics
backend that causes the buffer to fade when calling set_buffer, instead
of applying it immediately.

examples/rectangle.rs

@@ -0,0 +1,100 @@
+use softbuffer::GraphicsContext;
+use winit::event::{ElementState, Event, KeyboardInput, VirtualKeyCode, WindowEvent};
+use winit::event_loop::{ControlFlow, EventLoop};
+use winit::window::WindowBuilder;
+
+fn redraw(buffer: &mut [u32], width: usize, height: usize, flag: bool) {
+    for (index, color) in buffer.iter_mut().enumerate() {
+        let y = index / width;
+        let x = index % width;
+
+        if flag && x >= 100 && x < width - 100 && y >= 100 && y < height - 100 {
+            *color = 0x00ffffff;
+        } else {
+            let red = (x & 0xff) ^ (y & 0xff);
+            let green = (x & 0x7f) ^ (y & 0x7f);
+            let blue = (x & 0x3f) ^ (y & 0x3f);
+            *color = (blue | (green << 8) | (red << 16)) as u32;
+        }
+    }
+}
+
+fn main() {
+    let event_loop = EventLoop::new();
+
+    let window = WindowBuilder::new()
+        .with_title("Press space to show/hide a rectangle")
+        .build(&event_loop)
+        .unwrap();
+
+    #[cfg(target_arch = "wasm32")]
+    {
+        use winit::platform::web::WindowExtWebSys;
+
+        web_sys::window()
+            .unwrap()
+            .document()
+            .unwrap()
+            .body()
+            .unwrap()
+            .append_child(&window.canvas())
+            .unwrap();
+    }
+
+    let mut graphics_context = unsafe { GraphicsContext::new(&window, &window) }.unwrap();
+
+    let mut buffer = Vec::new();
+    let mut flag = false;
+
+    event_loop.run(move |event, _, control_flow| {
+        *control_flow = ControlFlow::Wait;
+
+        match event {
+            Event::RedrawRequested(window_id) if window_id == window.id() => {
+                // Grab the window's client area dimensions
+                let (width, height) = {
+                    let size = window.inner_size();
+                    (size.width as usize, size.height as usize)
+                };
+
+                // Resize the off-screen buffer if the window size has changed
+                if buffer.len() != width * height {
+                    buffer.resize(width * height, 0);
+                }
+
+                // Draw something in the offscreen buffer
+                redraw(&mut buffer, width, height, flag);
+
+                // Blit the offscreen buffer to the window's client area
+                graphics_context.set_buffer(&buffer, width as u16, height as u16);
+            }
+
+            Event::WindowEvent {
+                event: WindowEvent::CloseRequested,
+                window_id,
+            } if window_id == window.id() => {
+                *control_flow = ControlFlow::Exit;
+            }
+
+            Event::WindowEvent {
+                event:
+                    WindowEvent::KeyboardInput {
+                        input:
+                            KeyboardInput {
+                                state: ElementState::Pressed,
+                                virtual_keycode: Some(VirtualKeyCode::Space),
+                                ..
+                            },
+                        ..
+                    },
+                window_id,
+            } if window_id == window.id() => {
+                // Flip the rectangle flag and request a redraw to show the changed image
+                flag = !flag;
+                window.request_redraw();
+            }
+
+            _ => {}
+        }
+    });
+}