Fixing issue #32.

Added a check of SDL_GLContext;

src/gltests/include/testcommon.h:
  (1) added assert header when enabling debugging (check is on NDEBUG not being defined);

src/gltests/core-vao/main.c
src/gltests/fixedfunc-multitexcoord-2d/main.c
src/gltests/msaa_depthstencil/main.c
src/gltests/texture-array-1d/main.c
src/gltests/texture-array-2d-msaa/main.c
src/gltests/texture-array-2d/main.c
src/gltests/texture-array-nonzerobase/main.c
src/gltests/texture-array-subimages/main.c
src/gltests/texture-nonzerobase/main.c
  (1) added an assert on the SDL_GLContext being valid;
      why: Open source drivers currently support up to specific OpenGL context
           - so fail early (in order to avoid failure later in the code);