Make sure the width/height is large enough in SetOptionEncParamExt

When using 4 downsampled layers, make sure the random width/height
is large enough that the downsampled layers don't have a zero size.

Since the value never will be too small now, there's no need to
use WELS_CLIP3 any longer.
Merge pull request #1472 from mstorsjo/fix-test-random

Make sure the width/height is large enough in SetOptionEncParamExt