vo_gpu: fix vo=opengl legacy alias

Turns out the option code apparently tries to directly talloc_free() the
allocated strings, instead of going through a tactx wrapper or
something. So we can't directly overwrite it. Do something else
instead..

video/out/vo_gpu.c

@@ -47,6 +47,7 @@ struct gpu_priv {
     struct mp_log *log;
     struct ra_ctx *ctx;
 
+    bool force_gl; // for vo=opengl back-compat
     char *context_name;
     char *context_type;
     struct ra_ctx_opts opts;
@@ -289,7 +290,8 @@ static int preinit(struct vo *vo)
     struct ra_ctx_opts opts = p->opts;
     opts.want_alpha = alpha_mode == 1;
 
-    p->ctx = ra_ctx_create(vo, p->context_type, p->context_name, opts);
+    const char *type = p->force_gl ? "opengl" : p->context_type;
+    p->ctx = ra_ctx_create(vo, type, p->context_name, opts);
     if (!p->ctx)
         goto err_out;
     assert(p->ctx->ra);
@@ -361,7 +363,7 @@ static int preinit_opengl(struct vo *vo)
             " be removed in the future!\n");
 
     struct gpu_priv *p = vo->priv;
-    p->context_type = "opengl";
+    p->force_gl = true;
     return preinit(vo);
 }