vo_drm: don't mutate the current frame when clamping for panscan

When running with --panscan=1, this could crash - because the current
frame was reduced in size each time the image was redrawn, which would
result in a failed assertion the second time it's drawn.

(cherry picked from commit 859ddc9906)

video/out/vo_drm.c

@@ -491,12 +491,13 @@ static void draw_image(struct vo *vo, mp_image_t *mpi)
     struct priv *p = vo->priv;
 
     if (p->active) {
+        struct mp_image src = *mpi;
         struct mp_rect src_rc = p->src;
         src_rc.x0 = MP_ALIGN_DOWN(src_rc.x0, mpi->fmt.align_x);
         src_rc.y0 = MP_ALIGN_DOWN(src_rc.y0, mpi->fmt.align_y);
-        mp_image_crop_rc(mpi, src_rc);
-        mp_sws_scale(p->sws, p->cur_frame, mpi);
-        osd_draw_on_image(vo->osd, p->osd, mpi ? mpi->pts : 0, 0, p->cur_frame);
+        mp_image_crop_rc(&src, src_rc);
+        mp_sws_scale(p->sws, p->cur_frame, &src);
+        osd_draw_on_image(vo->osd, p->osd, src.pts, 0, p->cur_frame);
 
         struct modeset_buf *front_buf = &p->dev->bufs[p->dev->front_buf];
         int32_t shift = (p->device_w * p->y + p->x) * 4;