If image sizes are greater than 32768, fixed point stepping will overflow an int. This CL changes the max size to 32768 and disables the test if larger.

BUG=libyuv:590
TESTED=LIBYUV_FLAGS=-1 LIBYUV_WIDTH=8192 LIBYUV_HEIGHT=16 out/Release/libyuv_unittest --gtest_filter=*
R=harryjin@google.com

Review URL: https://codereview.chromium.org/1947783002 .