fix rescaling bug (uninitialized read, see bug #254).

the x_add/x_sub increments were wrong for u/v in the upscaling case.
They shouldn't be left to the caller's discretion, but set up by
WebPRescalerInit to their exact necessary values.

-> Cleaned-up WebPRescalerInit() param list.
-> added safety asserts
-> removed the mips32/mips_r2 variant of "ImportRow" which were buggy prior

Change-Id: I347c75804d835811e7025de92a0758d7929dfc09

src/enc/picture_rescale.c

@@ -181,10 +181,7 @@ static void RescalePlane(const uint8_t* src,
   int y = 0;
   WebPRescalerInit(&rescaler, src_width, src_height,
                    dst, dst_width, dst_height, dst_stride,
-                   num_channels,
-                   src_width, dst_width,
-                   src_height, dst_height,
-                   work);
+                   num_channels, work);
   memset(work, 0, 2 * dst_width * num_channels * sizeof(*work));
   while (y < src_height) {
     y += WebPRescalerImport(&rescaler, src_height - y,