refactor the PSNR / SSIM calculation code

-print_psnr is now much faster because it doesn't use the SSIM code.
The SSIM speed-up and re-write will come later.

Change-Id: Iabf565e0a8b41651d8164df1266cfeded4ab4823

src/dsp/dsp.h

@@ -250,7 +250,7 @@ extern VP8GetResidualCostFunc VP8GetResidualCost;
 void VP8EncDspCostInit(void);
 
 //------------------------------------------------------------------------------
-// SSIM utils
+// SSIM / PSNR utils
 
 // struct for accumulating statistical moments
 typedef struct {
@@ -275,6 +275,10 @@ typedef void (*VP8SSIMAccumulateFunc)(const uint8_t* src1, int stride1,
 extern VP8SSIMAccumulateFunc VP8SSIMAccumulate;         // unclipped / unchecked
 extern VP8SSIMAccumulateClippedFunc VP8SSIMAccumulateClipped;   // with clipping
 
+typedef uint32_t (*VP8AccumulateSSEFunc)(const uint8_t* src1,
+                                         const uint8_t* src2, int len);
+extern VP8AccumulateSSEFunc VP8AccumulateSSE;
+
 // must be called before using any of the above directly
 void VP8SSIMDspInit(void);