Set scissor on upscale to match camera viewport (bevyengine#14287)

# Objective

When the user renders multiple cameras to the same output texture, it
can sometimes be confusing what `ClearColorConfig` is necessary for each
camera to avoid overwriting the previous camera's output. This is
particular true in cases where the user uses mixed HDR cameras, which
means that their scene is being rendered to different internal textures.

## Solution

When a view has a configured viewport, set the GPU scissor in the
upscaling node so we don't overwrite areas that were written to by other
cameras.

## Testing

Ran the `split_screen` example.

crates/bevy_core_pipeline/src/upscaling/node.rs

@@ -84,6 +84,14 @@ impl ViewNode for UpscalingNode {
             .command_encoder()
             .begin_render_pass(&pass_descriptor);
 
+        if let Some(camera) = camera {
+            if let Some(viewport) = &camera.viewport {
+                let size = viewport.physical_size;
+                let position = viewport.physical_position;
+                render_pass.set_scissor_rect(position.x, position.y, size.x, size.y);
+            }
+        }
+
         render_pass.set_pipeline(pipeline);
         render_pass.set_bind_group(0, bind_group, &[]);
         render_pass.draw(0..3, 0..1);

examples/3d/split_screen.rs

@@ -68,12 +68,6 @@ fn setup(
                     camera: Camera {
                         // Renders cameras with different priorities to prevent ambiguities
                         order: index as isize,
-                        // Don't clear after the first camera because the first camera already cleared the entire window
-                        clear_color: if index > 0 {
-                            ClearColorConfig::None
-                        } else {
-                            ClearColorConfig::default()
-                        },
                         ..default()
                     },
                     ..default()