add split_screen example, with 1, 2, or 4 cameras

[RJ]

Added a split_screen example, based on dynamic example.

It defaults to QuadSplit with 4 cameras, which works fine and you can toggle between camera layouts with F1.

However, if you change the default to Single it crashes when changing to VerticalSplit:

// TODO with Single as default, crashes when changing to VerticalSplit.
// (and if 2-player vertical split as default, crashes when changing to quad)
#[derive(Resource, Debug, Default, Clone, PartialEq, Eq, Hash, Reflect)]
pub enum CameraLayout {
    /// Single player game
    Single,
    /// 2 player vertical split game
    VerticalSplit,
    /// 4 player split
    #[default]
    QuadSplit,
}

this is a reproduction in example form of #14

2025-04-25T10:24:43.624996Z  INFO split_screen: Toggling camera layout
2025-04-25T10:24:43.625023Z  INFO split_screen: Despawning 1 cameras
2025-04-25T10:24:43.625026Z  INFO split_screen: Spawning cameras for VerticalSplit
2025-04-25T10:24:43.625028Z  INFO split_screen: Spawning 2 cameras
2025-04-25T10:24:43.625114Z  INFO split_screen: Setting camera viewports
2025-04-25T10:24:43.625129Z  INFO split_screen: Setting viewport for camera 1: Viewport { physical_position: UVec2(0, 0), physical_size: UVec2(1280, 1440), depth: 0.0..1.0 }
2025-04-25T10:24:43.625135Z  INFO split_screen: Setting viewport for camera 2: Viewport { physical_position: UVec2(1280, 0), physical_size: UVec2(1280, 1440), depth: 0.0..1.0 }
2025-04-25T10:24:43.630858Z ERROR wgpu::backend::wgpu_core: Handling wgpu errors as fatal by default    

thread '' panicked at /Users/rj/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wgpu-24.0.3/src/backend/wgpu_core.rs:3273:26:
wgpu error: Validation Error

Caused by:
  In RenderPass::end
    In a set_bind_group command
      Dynamic binding offset index 0 with offset 2304 would overrun the buffer bound to BindGroup with 'particle_view_bind_group' label 0 -> binding 0. Buffer size is 1536 bytes, the binding binds bytes 0..752, meaning the maximum the binding can be offset is 784 bytes


note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
2025-04-25T10:24:43.631200Z ERROR wgpu::backend::wgpu_core: Handling wgpu errors as fatal by default    

thread 'Compute Task Pool (0)' panicked at /Users/rj/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/wgpu-24.0.3/src/backend/wgpu_core.rs:3273:26:
wgpu error: Validation Error

Caused by:
  In RenderPass::end
    In a set_bind_group command
      Dynamic binding offset index 0 with offset 1536 would overrun the buffer bound to BindGroup with 'particle_view_bind_group' label 0 -> binding 0. Buffer size is 1536 bytes, the binding binds bytes 0..752, meaning the maximum the binding can be offset is 784 bytes


Encountered a panic in system `bevy_render::renderer::render_system`!