vo_opengl: fix backend autoprobing

We want the following behavior:
- VO probed, backend probed: only accept non-sw, fail completely
otherwise
- VO forced, backend probed: use the first non-sw, or if none is found,
fall back to the first working sw backend
- VO probed, backend forced: (I don't care about this case)
- VO forced, backend forced: just use that backend

Also, on backend probe failure the vo->probed field was left in its old
state.

video/out/opengl/common.c

@@ -597,6 +597,7 @@ static MPGLContext *init_backend(struct vo *vo, const struct mpgl_driver *driver
     MP_VERBOSE(vo, "Initializing OpenGL backend '%s'\n", ctx->driver->name);
     ctx->priv = talloc_zero_size(ctx, ctx->driver->priv_size);
     if (ctx->driver->init(ctx, vo_flags) < 0) {
+        vo->probing = old_probing;
         talloc_free(ctx);
         return NULL;
     }
@@ -639,6 +640,14 @@ MPGLContext *mpgl_init(struct vo *vo, const char *backend_name, int vo_flags)
             if (ctx)
                 break;
         }
+        // VO forced, but no backend is ok => force the first that works at all
+        if (!ctx && !vo->probing) {
+            for (int n = 0; n < MP_ARRAY_SIZE(backends); n++) {
+                ctx = init_backend(vo, backends[n], false, vo_flags);
+                if (ctx)
+                    break;
+            }
+        }
     } else if (index >= 0) {
         ctx = init_backend(vo, backends[index], false, vo_flags);
     }