bgfx_path bgfx
bgfx_backend auto
bgfx_debug 0
bgfx_screen_chains crt-geom,crt-geom,crt-geom
bgfx_shadow_mask slot-mask.png
bgfx_avi_name bgfx.avi

Not showing the effect here on MAME64 (Windows 10 with AMD graphics card). I just tried it out. I'm doing it wrong? The rest of the bgfx shaders works.

Windows 10 Home 64-bit / AMD Radeon RX 5700 XT / AMD Ryzen 7 3700X 8-Core 3.59 GHz / RAM 16 GB