revert accidental commits, assert for zero-sized build_vertex_buffer(), explain the TODO on no-clear on skybox present

src/rendering/twn_gl_any_rendering.c

@@ -70,6 +70,9 @@ void clear_draw_buffer(void) {
                  (1.0f / 255) * 230,
                  (1.0f / 255) * 230, 1);
 
+    /* TODO: don't clear color when skybox is applied? */
+    /*       for that window should match framebuffer */
+    /*       also it is driver dependent, from what i can gather */
     glClear(GL_COLOR_BUFFER_BIT |
             GL_DEPTH_BUFFER_BIT |
             GL_STENCIL_BUFFER_BIT);