Changes besides the port to SDL2:

- Use SDL_opengles2.h instead of GLEW.
- Force an OpenGL ES 2.0 profile.
- Remove the depth buffer selection method, as this is not an OpenGL ES
  2.0 feature.

To remove the dependency on GLEW, a copy of shader_utils.cpp has been
added that also only depends on SDL_opengles2.h.