Allow transparet background in WidgetScrollBox

Transparency is true by default. Please note that when rendering other
widgets inside of a transparent WidgetScrollBox, the `render_to_alpha`
flag must be set to true for those widgets.
Merge pull request #564 from dorkster/bugfixes

Allow transparet background in WidgetScrollBox
Don't process expired hazards

Fixes a segfault caused by expired hazards (aka hazards that have
traveled outside the map).