gen_scalers: fix 64-bit integer promotion bug

i needs to be treated as signed to get the proper indexing on 64-bit
platforms. This behavior was accidentally reverted when fixing an
unsigned/signed comparison warning.

Change-Id: Ic306d609bdc8de94c8f8ba29c6e45c736101a82e

vpx_scale/generic/gen_scalers.c

@@ -937,12 +937,13 @@ void vp8cx_vertical_band_2_1_scale_c(unsigned char *source, unsigned int src_pit
 
 void vp8cx_vertical_band_2_1_scale_i_c(unsigned char *source, unsigned int src_pitch, unsigned char *dest, unsigned int dest_pitch, unsigned int dest_width)
 {
-    unsigned int i;
+    int i;
     int temp;
+    int width = dest_width;
 
     (void) dest_pitch;
 
-    for (i = 0; i < dest_width; i++)
+    for (i = 0; i < width; i++)
     {
         temp = 8;
         temp += source[i-(int)src_pitch] * 3;