snapdiff: Provide indication whether images match or not
snapdiff: Provide indication whether images match or not

This indication is provided in two ways:

1. The exit value of the script will be 1 if there are any image
   differences, (0 for no image differences).

2. The filename cell in the resulting HTML table will be colored red
   if there are image differences (and green for no image
   differences).

Signed-off-by: José Fonseca <3bcec90dbf5a68342d86695ff042005b96b780f5@gmail.com>

surround-io/apitrace,EoD/apitrace,apitrace/apitrace,PeterLValve/apitrace,surround-io/apitrace,EoD/apitrace,joshua5201/apitrace,trtt/apitrace,trtt/apitrace,EoD/apitrace,schulmar/apitrace,apitrace/apitrace,joshua5201/apitrace,tuanthng/apitrace,joshua5201/apitrace,apitrace/apitrace,swq0553/apitrace,swq0553/apitrace,PeterLValve/apitrace,swq0553/apitrace,PeterLValve/apitrace,tuanthng/apitrace,EoD/apitrace,trtt/apitrace,schulmar/apitrace,surround-io/apitrace,schulmar/apitrace,surround-io/apitrace,joshua5201/apitrace,joshua5201/apitrace,schulmar/apitrace,EoD/apitrace,swq0553/apitrace,tuanthng/apitrace,schulmar/apitrace,trtt/apitrace,tuanthng/apitrace,PeterLValve/apitrace,tuanthng/apitrace,apitrace/apitrace,trtt/apitrace,surround-io/apitrace,swq0553/apitrace