This adds gs_enum_adapters and gs_get_adapter_count to the opengl
backend and promotes these to multiplatform graphics functions.
However we need to make an internal device change, device_enum_adapters
must pass in the current device on opengl to ensure that adapter #0 is
the display adapter. We do this to avoid changes to plugins already
checking against obs_video_info.adapter which is always 0 and expected
to be the device OBS was initialized on.
The actual implementation reports the dri render node (or /Software).
This allows plugins to query non-video features of the adapters like
VA-API/NVENC/etc or other cross device functionality. `/Software` is
chosen to avoid opening random files in the current directory if its
passed along as a file path like the regular dri render nodes.
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