glClear the FBO in debug builds

Prior to the 'Get rid of vbo flipping' commit, this would cause
every second frame to be very obviously broken on certain drivers.
This ensures that if we regress the render target, I'll see it in
a debug build.

src/video/video.c

@@ -207,6 +207,10 @@ void video_render_finish_offscreen(void) {
     activate_program(g_video_state.palette_prog_id);
     render_target_activate(g_video_state.target);
 
+#if DEBUGMODE
+    glClear(GL_COLOR_BUFFER_BIT);
+#endif
+
     object_array_blend_mode mode;
     while(object_array_get_batch(g_video_state.objects, &batch, &mode)) {
         video_set_blend_mode(mode);