Add simple env. var. for FPS limit

[Tiagoquix]

Like DXVK_FRAME_RATE, add a simple env. var. to toggle the MangoHUD FPS limit.

Such as: MANGOHUD_FRAME_RATE or MANGOHUD_FPS_LIMIT.

The user configuration will load normally, but the env. var. would take precedence (in regards to the FPS limit only). It would be the same effect as changing the fps_limit line in the config. file.

Thanks!

[Leopard1907]

MANGOHUD_CONFIG=read_cfg,fps_limit=numberyouwant

[Tiagoquix]

Interesting, thanks for letting me know!

I still think a single env. var. for that could be added. :)