main: allow overriding the backend with the value of a ConVar

[oSoMoN]

No description provided.

[misyltoad]

id rather just factor this all out into cmdline + env vars automatically setting convars

[oSoMoN]

id rather just factor this all out into cmdline + env vars automatically setting convars

I reworked the patch to expose the backend as a ConVar, which is good enough for my needs, no need for an environment variable.

@misyltoad if I understand correctly your suggestion, any base ConVar could be set from the value of an environment variable suitably named? E.g. for all environment variables whose name starts with GAMESCOPE_CONVAR_, figure out the corresponding ConVar and set its value. Is that correct? If so I can look into implementing that in a separate PR.

[oSoMoN]

@misyltoad if I understand correctly your suggestion, any base ConVar could be set from the value of an environment variable suitably named? E.g. for all environment variables whose name starts with GAMESCOPE_CONVAR_, figure out the corresponding ConVar and set its value. Is that correct? If so I can look into implementing that in a separate PR.

I gave a go at this in #2163. Feedback welcome!