vo_opengl: make ra_fns.timer_create optional

video/out/opengl/ra.h

@@ -382,7 +382,7 @@ struct ra_fns {
                            const struct ra_renderpass_run_params *params);
 
     // Create a timer object. Returns NULL on failure, or if timers are
-    // unavailable.
+    // unavailable for some reason. Optional.
     ra_timer *(*timer_create)(struct ra *ra);
 
     void (*timer_destroy)(struct ra *ra, ra_timer *timer);

video/out/opengl/utils.c

@@ -134,6 +134,9 @@ struct timer_pool {
 
 struct timer_pool *timer_pool_create(struct ra *ra)
 {
+    if (!ra->fns->timer_create)
+        return NULL;
+
     ra_timer *timer = ra->fns->timer_create(ra);
     if (!timer)
         return NULL;