vo_gpu: opengl: do not free "GL" sub-allocations

This function always expects the GL struct pointer to be a talloc
allocation. So far so bad. But the terrible thing is that _lots_ of code
in mpv didn't quite get this (including the code which introduced the
way it is used this way). For example, in context_glx.c you see this:

struct priv {
    GL gl;
    ...

GL is not a talloc allocation, but since it's at the start of a talloc
allocation, it works anyway. So far so bad. But the really terrible
thing is that mpgl_load_functions2() calls talloc_free_children() on the
GL pointer, which means that all of priv's. This would be unintentional
and could create dangling pointers. And this happens at the about 1
dozen of callers. I'm amazed it didn't broke yet anywhere.

Removing this anti-pattern with making GL "implicitly" a talloc
allocation would be too much effort at this point. So just manually free
the only allocation that the function attached to GL.

video/out/opengl/common.c

@@ -494,7 +494,7 @@ static const struct gl_functions gl_functions[] = {
 void mpgl_load_functions2(GL *gl, void *(*get_fn)(void *ctx, const char *n),
                           void *fn_ctx, const char *ext2, struct mp_log *log)
 {
-    talloc_free_children(gl);
+    talloc_free(gl->extensions);
     *gl = (GL) {
         .extensions = talloc_strdup(gl, ext2 ? ext2 : ""),
     };