OrthographicProjection: place origin at integer pixel with WindowSize…

… scaling mode (#4085)

# Objective

One way to avoid texture atlas bleeding is to ensure that every vertex is
placed at an integer pixel coordinate. This is a particularly appealing
solution for regular structures like tile maps.

Doing so is currently harder than necessary when the WindowSize scaling
mode and Center origin are used: For odd window width or height, the
origin of the coordinate system is placed in the middle of a pixel at
some .5 offset.

## Solution

Avoid this issue by rounding the half width and height values.

crates/bevy_render/src/camera/projection.rs

@@ -104,12 +104,14 @@ impl CameraProjection for OrthographicProjection {
     fn update(&mut self, width: f32, height: f32) {
         match (&self.scaling_mode, &self.window_origin) {
             (ScalingMode::WindowSize, WindowOrigin::Center) => {
-                let half_width = width / 2.0;
-                let half_height = height / 2.0;
-                self.left = -half_width;
+                let half_width = (width / 2.0).round();
+                let half_height = (height / 2.0).round();
+                // Assign left and bottom such that (right-left)==width and (top-bottom)==height
+                // still hold with  with rounded half_width and half_height
+                self.left = half_width - width;
                 self.right = half_width;
                 self.top = half_height;
-                self.bottom = -half_height;
+                self.bottom = half_height - height;
             }
             (ScalingMode::WindowSize, WindowOrigin::BottomLeft) => {
                 self.left = 0.0;