Fix int64_t to unsigned int conversion warnings

Use unsigned int type to store the sse in the pixel domain. The
precision is sufficient to handle sse of block size up to 64x64.
The transform domain version however needs int64_t, since there is
a transfer gain applied in the forward transformation that might
cause unsigned int overflow.

Change-Id: Ifef97c38597e426262290f35341fbb093cf0a079

vp9/encoder/vp9_rdopt.c

diff --git a/vp9/encoder/vp9_rdopt.c b/vp9/encoder/vp9_rdopt.c
index d3a1db8..c55f1e4 100644
--- a/vp9/encoder/vp9_rdopt.c
+++ b/vp9/encoder/vp9_rdopt.c
@@ -172,7 +172,7 @@
   const int ref = xd->mi[0]->mbmi.ref_frame[0];
   unsigned int sse;
   unsigned int var = 0;
-  int64_t sum_sse = 0;
+  unsigned int sum_sse = 0;
   const int shift = 8;
   int rate;
   int64_t dist;