VSE: Fix incorrect cache memory usage calculation

If image is cached twice, it's size has been counted twice as well, but
only image reference count is increased, not memory usage.

Use `MEM_get_memory_in_use()` instead of size own tracking.

source/blender/sequencer/intern/image_cache.h

@@ -65,7 +65,7 @@ void seq_cache_cleanup_sequence(struct Scene *scene,
                                 struct Sequence *seq_changed,
                                 int invalidate_types,
                                 bool force_seq_changed_range);
-bool seq_cache_is_full(struct Scene *scene);
+bool seq_cache_is_full(void);
 
 #ifdef __cplusplus
 }