creation of opengl 1.5 context, mixing of gl calls with sdl renderer

src/context.h

@@ -50,6 +50,7 @@ typedef struct context {
     /* it can be changed at runtime; any resulting logic anomalies are bugs */
     unsigned int update_multiplicity;
 
+    SDL_GLContext *gl_context;
     SDL_Renderer *renderer;
     SDL_Window *window;
     uint32_t window_id;