Reverted back to not using Graphics class

Separate Graphics class was an experiment made in anticipation of migrating to Allegro 5 library and meant to ease adopting peculiarities of its logic. But since Allegro 5 is still under question, it provides no significant benefit but rather complications.
Common: revised several safety checks in stream classes
Merge branch 'master' of github.com:adventuregamestudio/ags