Linux Fix BGFX bug with multihead MAME freezing on exit

[substring]

Fix bug with MAME freezing when exiting from the 3rd fullscreen window with -video bgfx. Probably linked to library release on exit.

[D0023R]

Command line to reproduce:

mamearcade64 -seconds_to_run 10 -m arcade_31 -verbose -switchres -nomodesetting -numscreens 3 -video bgfx darius

The application freezes after "Enter sdlwindow_exit".

Using X11 video driver the issue is not present.

Specifying 2 screens results in proper restoration of desktop and sdl finalization but execution ends with a "Segmentation fault (core dumped)". This result is identical when a single or more screens game spread over more than 2 physical monitors (-video bgfx -numscreens 2+).