Change buffer_alloc_sz and frame_size type to size_t

1. Changed buffer_alloc_sz and frame_size type to size_t.
2. Added a TODO for video resolution limits. On 32 bit systems, the maximum
resolution supported in the encoder is 4k(3840x2160). The malloc() would
fail if encoding >4k video on a 32 bit system.

Change-Id: Ibd91b28fd63d1b04e8ac9a5270a17629f239188a
diff --git a/test/hbd_metrics_test.cc b/test/hbd_metrics_test.cc
index f8c0517..abe82be 100644
--- a/test/hbd_metrics_test.cc
+++ b/test/hbd_metrics_test.cc
@@ -96,7 +96,7 @@
   void RunAccuracyCheck() {
     const int width = 1920;
     const int height = 1080;
-    int i = 0;
+    size_t i = 0;
     const uint8_t kPixFiller = 128;
     YV12_BUFFER_CONFIG lbd_src, lbd_dst;
     YV12_BUFFER_CONFIG hbd_src, hbd_dst;