v2.2-alpha 'default' overlay for every game

Updated version that changes steamgrid to include any game that it finds - tagged or not - and add a 'default' tag to it.
Thus, the default.png overlay will be applied to every game found.

The default.png is just empty right now and forces Steam to cache grid images as-is to mitigate the bug mentioned in this Steam Forum Thread.
But it could easily be changed to apply a 'vignette' effect etc. to every game image found if desired.