use Over as compose argument for dummy sprites
use Over as compose argument for dummy sprites

'Over' is an equivalent option to 'src-over' for composite, introduced
at some point before Imagemagick 6.x.

The 'graphicsmagick' fork of Imagemagick provides a (supposedly)
compatible imagemagick interface, but lacks the (older) 'src-over'
composite type.

Using 'Over' means you can build with imagemagick or graphicsmagick.

Signed-off-by: Mike Swanson <517655fde76fa2f07143b6c6f1774e4fae45930a@gmail.com>

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