NEON optimised yuv to rgb conversion

- along the lines of the SSE chroma upsampling.
Total speedup is ~30%.

4% speed loss on YuvToRgbXX conversion using tables instead
of 14-bit fixed precision. TODO(later): investigate, and compare
to x86.

see http://code.google.com/p/webp/issues/detail?id=134

Change-Id: Idc2261037cd13b4553ca20ecc4c4007099c37009

src/dsp/dsp.h

@@ -158,6 +158,9 @@ extern WebPUpsampleLinePairFunc WebPUpsamplers[/* MODE_LAST */];
 // Initializes SSE2 version of the fancy upsamplers.
 void WebPInitUpsamplersSSE2(void);
 
+// NEON version
+void WebPInitUpsamplersNEON(void);
+
 #endif    // FANCY_UPSAMPLING
 
 // Point-sampling methods.
@@ -199,6 +202,7 @@ extern void (*WebPApplyAlphaMultiply4444)(
 void WebPInitPremultiply(void);
 
 void WebPInitPremultiplySSE2(void);   // should not be called directly.
+void WebPInitPremultiplyNEON(void);
 
 //------------------------------------------------------------------------------