Function source_over 

pub const fn source_over(dst: u32, src_premul: u32, alpha: u32) -> u32

Composites a premultiplied source over a destination pixel.

Both words are 0xAARRGGBB. The alpha lane is composited too, so this is correct for an Argb8888 target as well as an opaque Xrgb8888 one.