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/utils/rescaler.h

@@ -46,8 +46,6 @@ void WebPRescalerInit(WebPRescaler* const rescaler,
                       uint8_t* const dst,
                       int dst_width, int dst_height, int dst_stride,
                       int num_channels,
-                      int x_add, int x_sub,
-                      int y_add, int y_sub,
                       int32_t* const work);
 
 // Returns the number of input lines needed next to produce one output line,