Since the list of available GPU contexts is a compile time constant,
use obj_settings_list instead of opt_string_validate for GPU contexts.
This has several advantages:
- Aligns with the existing usage of vo, ao, and filter lists.
- Allows custom probing order.
- Allows list option suffixes. (--gpu-context-append, etc.)
- Allows autocomplete in console.lua.
- Uses the standard obj_settings_list help printing, so the custom
help printing function is no longer needed.
This also deduplicates some context creation code for ra_ctx_create
and ra_ctx_create_by_name.
nanahi
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