SSE2: yuv->rgb speed-up for point-sampling

- use statically initialized tables (if WEBP_YUV_USE_SSE2_TABLES is defined)
 - use SSE2 row conversion for yuv->ARGB / RGBA / ABGR / RGB / BGR
 - clean-up and harmonize the WebpUpsamplers[] usage.

Change-Id: Ic5f3659a995927bd7363defac99c1fc03a85a47d

src/dsp/yuv.h

@@ -245,6 +245,10 @@ void VP8YUVInit(void);
 
 #if defined(WEBP_USE_SSE2)
 
+// When the following is defined, tables are initialized statically, adding ~12k
+// to the binary size. Otherwise, they are initialized at run-time (small cost).
+#define WEBP_YUV_USE_SSE2_TABLES
+
 #if defined(FANCY_UPSAMPLING)
 // Process 32 pixels and store the result (24b or 32b per pixel) in *dst.
 void VP8YuvToRgba32(const uint8_t* y, const uint8_t* u, const uint8_t* v,