The Block object
These objects form the individual pieces of the _terrain object. They can take the shape of a building, or an empty gap between buildings. We'll have four different types of buildings, which later will represent four different types of textures when we finally bring in our sprite sheets. These blocks can have different widths and heights.
Block also derives from GameSprite and it also has inline helper methods to retrieve information about its boundaries, but only in relation to its current position, since Block doesn't technically move.
