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.cpp

@@ -26,10 +26,11 @@ extern "C" {
 
 OLIVE_NAMESPACE_ENTER
 
-FFmpegFramePool::FFmpegFramePool() :
-  width_(0),
-  height_(0),
-  format_(AV_PIX_FMT_NONE)
+FFmpegFramePool::FFmpegFramePool(int element_count, int width, int height, AVPixelFormat format) :
+  MemoryPool(element_count),
+  width_(width),
+  height_(height),
+  format_(format)
 {
 }
 
@@ -51,34 +52,8 @@ FFmpegFramePool::ElementPtr FFmpegFramePool::Get(AVFrame *copy)
   return ele;
 }
 
-void FFmpegFramePool::SetParams(int width, int height, AVPixelFormat format)
-{
-  int old_nb_elements;
-
-  if (IsAllocated()) {
-    old_nb_elements = GetElementCount();
-
-    Destroy();
-  } else {
-    old_nb_elements = 0;
-  }
-
-  width_ = width;
-  height_ = height;
-  format_ = format;
-
-  if (old_nb_elements) {
-    // Re-allocate automatically
-    Allocate(old_nb_elements);
-  }
-}
-
 size_t FFmpegFramePool::GetElementSize()
 {
-  if (width_ == 0 || height_ == 0 || format_ == AV_PIX_FMT_NONE) {
-    return 0;
-  }
-
   int buf_sz = av_image_get_buffer_size(static_cast<AVPixelFormat>(format_),
                                         width_,
                                         height_,