Fix overflow in `RenderLayers::iter_layers`

[Azorlogh]

Objective

• Fixes overflow when calling RenderLayers::iter_layers on layers of the form k * 64 - 1
  • Causes a panic in debug mode, and an infinite iterator in release mode

Solution

• Use u64::checked_shr instead of >>=

Testing

• Added a test case for this: render_layer_iter_no_overflow