Make sure shadow is removed when game is over

[alpgarcia]

Sometimes part of the shadow will be still displayed after game over. This happens when the shape's new position is not valid and is less than the END_POS. The shape won't be placed on the grid, but part of the shadow could still be there because the shapes start falling from above the grid.

We could either remove those grid cells having their Color.alpha value distinct from 1 (that means they are transparent and that means they are part of the shadow) or try to make sure we only draw a shadow if the whole shadow is on the visible grid.