Using "SDL2/SDL_xxx.h" instead of "SDL_xxx.h" caused
the include paths provided by pkg-config to be ignored,
and headers from a standard include path to be used instead.
The drawing is now completely shader based, which makes away
with all usage of the depracted matrix stack. This also allows
us to do things like simple translations and texture coordinate
translation directly instead of doing everything indirectly
through matrices.
Fixed vertex attributes ('vertexPointer()' etc) are also
replaced with user defined attribute arrays.
Jonas Kulla