Wrong include paths in SDL2 examples

Includes for SDL2 headers should be without the SDL2 directory itself, `sdl2-config --cflags` returns the correct include directory. I noticed this when I added this repository as a submodule and tried including the examples in my own project. I don't use the system installed SDL2, so it fails to include.

Start by locating the SDL2 examples and inspect their SDL2 header include paths alongside the include directory returned by `sdl2-config --cflags`. Done means the examples use paths compatible with a submodule setup without requiring system-installed SDL2 headers.

Written by the indexing model from the issue text.