Thanks! It's hard to debug this sort of thing on Linux.
I also added in a magenta/green striped aperture mask just because I thought it was a standard feature in most crt-geom versions floating around the internet. And I added a slider to turn off the effect. I hope cgwg doesn't mind...
The main reason I never included this in the MAME version is that the same functionality was available using the -effect overlays.
I've created a new version that does a similar thing using textures loaded from files, and I've created a bunch of suitable images. Note that these are all designed for displays with RGB subpixel order, so they won't behave that well under rotation. It's available here
, again in uncompiled form.