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.
Avoid warnings about converting from int to bool

MSVC used to warn about this.
Add a cast when converting from int64_t to float

This avoids a warning when building with MSVC.
add validation of slice parameters instead of assert
Merge pull request #1472 from mstorsjo/fix-test-random

Make sure the width/height is large enough in SetOptionEncParamExt
Merge pull request #1478 from sijchen/after_review61

[Encoder] add validation of slice parameters instead of assert
Merge pull request #1477 from mstorsjo/avoid-msvc-warnings

Avoid warnings when building with MSVC
Set constraint set 0 and 1 flags for non-scalable as well

This allows decoders to know that a baseline stream doesn't
contain any of the features that aren't in the constrained
baseline subset.

Set bConstraintSet0Flag if using baseline profile (not using
cabac). Always set bConstraintSet1Flag since we always follow
the restrictions of the main profile.