Fix unreported: Sequencer grid lines are not drawn

By design, there should be lines between the alternating horizontal
stripes in the Sequencer. But currently they are all drawn in one place,
on top of each other. Mistake in rBfae895125efe.

Differential Revision: https://developer.blender.org/D9962

source/blender/editors/space_sequencer/sequencer_draw.c

@@ -1911,6 +1911,7 @@ static void draw_seq_backdrop(View2D *v2d)
   while (line_len--) {
     immVertex2f(pos, v2d->cur.xmax, i);
     immVertex2f(pos, v2d->cur.xmin, i);
+    i++;
   }
   immEnd();