Start abstracting the rendering backend away to make it easier to switch to GPU rendering later

src/draw.c

@@ -20,11 +20,11 @@ void akgl_draw_background(int w, int h)
         for (x = 0; x < w; x += dx) {
             /* use an 8x8 checkerboard pattern */
             i = (((x ^ y) >> 3) & 1);
-            SDL_SetRenderDrawColor(renderer, col[i].r, col[i].g, col[i].b, col[i].a);
+            SDL_SetRenderDrawColor(renderer.sdl_renderer, col[i].r, col[i].g, col[i].b, col[i].a);
 
             rect.x = (float)x;
             rect.y = (float)y;
-            SDL_RenderFillRect(renderer, &rect);
+            SDL_RenderFillRect(renderer.sdl_renderer, &rect);
         }
     }
 }