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/dec/vp8l.c

@@ -441,8 +441,7 @@ static int AllocateAndInitRescaler(VP8LDecoder* const dec, VP8Io* const io) {
   scaled_data = (uint32_t*)memory;
 
   WebPRescalerInit(dec->rescaler, in_width, in_height, (uint8_t*)scaled_data,
-                   out_width, out_height, 0, num_channels,
-                   in_width, out_width, in_height, out_height, work);
+                   out_width, out_height, 0, num_channels, work);
   return 1;
 }