libobs: Add function to enumerate video adapters

The gs_enum_adapters function is an optional implementation to allow
enumeration of available graphics adapters that can be used with the
program.  The ID associated with the adapter can be an index or a hash
depending on the implementation.

libobs/graphics/graphics-imports.c

@@ -42,6 +42,7 @@ bool load_graphics_imports(struct gs_exports *exports, void *module,
 
 	GRAPHICS_IMPORT(device_get_name);
 	GRAPHICS_IMPORT(device_get_type);
+	GRAPHICS_IMPORT_OPTIONAL(device_enum_adapters);
 	GRAPHICS_IMPORT(device_preprocessor_name);
 	GRAPHICS_IMPORT(device_create);
 	GRAPHICS_IMPORT(device_destroy);