WebPRescalerInit: add missing int64_t promotion
WebPRescalerInit: add missing int64_t promotion

large values of x_add and y_add may rollover an int causing a later
assertion to fail in WebPRescalerExportRow due to fxy_scale incorrectly
being set to 0.

fixes:
src/dsp/rescaler.c:178: void WebPRescalerExportRow(WebPRescaler *const):
Assertion `wrk->src_height == wrk->dst_height && wrk->x_add == 1'
failed.

Bug: chromium:1196480
Change-Id: I2c00f015d61a1257033d8edb1edd4d060d6878b7

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp