glimagesink: Fix render rect assertion
Whenever the surface is resized before the stream is negotiated, we endup with an assertion in libgstvideo.
gst_video_center_rect: assertion 'src->h != 0' failed
This fixes it, by following the style aready in place, which is to ensure surfaces have a minimum size of 1x1.
Fixes #1139 (closed)
p.s. I can't reproduce this, don't merge until someone who can reproduce can verify