memorypool: implemented arenas

A pool can allocate/destroy any amount of arenas during runtime, meaning it
won't run out of memory but we still get the advantages of one large allocation
as opposed to hundreds of small allocations.

app/codec/ffmpeg/ffmpegframepool.h

@@ -30,12 +30,13 @@ OLIVE_NAMESPACE_ENTER
 class FFmpegFramePool : public MemoryPool<uint8_t>
 {
 public:
-  FFmpegFramePool();
+  FFmpegFramePool(int element_count,
+                  int width,
+                  int height,
+                  AVPixelFormat format);
 
   ElementPtr Get(AVFrame* copy);
 
-  void SetParams(int width, int height, AVPixelFormat format);
-
 protected:
   virtual size_t GetElementSize() override;