Allow customising ray step size in render window

## Use Case

When I'm rendering a sample, I'll often set the primary and secondary step size to ~10 to make sure performance is good enough when I'm tweaking options in the main window. Then when I go to render, I switch the step size to 1 to get a better final rendering. But this requries setting the step size, then going to the render window, then resetting it to 10 when the render is done.

## Solution

Instead, it would be nice to customize the step size in the render window and have it not affect the step size in the main window.

## Alternatives

I can't think of any other alternatives, although I could understand not implementing this to keep the rendering window simple.