Function apply_drop_shadow 

fn apply_drop_shadow(
    pixmap: &mut Pixmap,
    dx: f32,
    dy: f32,
    std_deviation: f32,
    n_decimations: usize,
    kernel: &[f32],
    color: AlphaColor<Srgb>,
    edge_mode: EdgeMode,
    composite_original: bool,
    filter_scratch: &mut ScratchBuffer,
)

Apply drop shadow effect.

This is the main entry point that splits the drop shadow operation into well-defined steps:

1. Offset the shadow pixels
2. Blur the already-offset shadow
3. Apply shadow color and optionally composite with original