Tilemap: Optimize scanrow z ordering
Tilemap: Draw consecutive scanrows in one draw call

If consecutive scanrows in the scene list have no foreign
elements in between them, we batch them up and draw them
in one glDrawElements() call.

This should reduce the Tilemap induced draw calls on
average by at least 50 percent.
Remove useless SDL_Init() flag
Fix up SDL2 include paths

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.
Fix up glew include paths