Redirect stderr to log file in vm_run_lua_on_skps.sh

This is to help diagnoze the below bug.

BUG=skia:3117
TBR=

Review URL: https://codereview.chromium.org/723613002
This ignores the alpha channel when comparing images.

BUG=skia:2930
TBR=rmistry

Review URL: https://codereview.chromium.org/718133002