MAME 0.215 - clear BGFX Performance Improvements (WIP)

Yesterday MAME 0.215 came out, and the Changelog also refers to changes in the BGFX shader. While the main change, the implementation of a Vulcan shader, is entirely unrelated to MacOS, my initial performance tests with the usual candidates show massive performance improvements.

System: MacOS Mojave 10.14.6
MAME: official build 0.215 (http://sdlmame.lngn.net/)
Terminal command: ./mame64 -window -video bgfx -str 60 -noafs -bgfx_screen_chains crt-geom-deluxe -bgfx_backend metal -noreadconfig -nosleep -nothrottle (ROMFILENAME

Example 1942 (Revision B): 765.06% in MAME 0.201, 907,94% in MAME 0.215

Details to follow, I hope.