Make DepthOfField default respect DepthOfFieldMode default

[FindlayRoyds]

Objective

The default value for DepthOfFieldMode is Gaussian, but the default implementation of DepthOfField uses Bokeh.

Old version:

pub enum DepthOfFieldMode {
    /// ...
    Bokeh,

    /// ...
    /// This is the default.
    /// ...
    #[default]
    Gaussian,
}

impl Default for DepthOfField {
    fn default() -> Self {
        // ...
        Self {
            // ...
            mode: DepthOfFieldMode::Bokeh,
        }
    }
}

Solution

This PR updates DepthOfField to use the default value of DepthOfFieldMode.

Testing

The depth of field example compiles and runs. I adjusted it to use the default mode of DepthOfField and it correctly uses Gaussian now.

[github-actions]

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