Removed SDL2::Event#type_symbol, no longer needed.
SDL2::TTF::Font now has a ::default_fg and ::default_bg attribute.
Updated LazyFoo 07 spec
Commented out debugging puts
SDL2::Window#position now relies on SDL2::Struct#free
Struct#== now matches "partial hashes"
Struct::cast() now uses Struct#update_members to assign values.
A first attempt at an application framework.
Event#cast can now handle partial hashes.
Point::cast_array has been implemented as a prototype.  It may be used
for Struct::cast_array
Removed Rect#equals() in favor of Struct#==
Implemented Rect#has_intersection?, #intersection, #union,
#enclose_points, #intersect_line
Implemented new Lazy Foo Tutorial: 09 Mouse Events