Normally, the DownsamplePadding skips scaling if the target
size is the same as the source size, assuming that the caller
will use the source data pointer in that case. This is true
for the base layer (the first call to DownsamplePadding in
SingleLayerPreprocess), but when downsampling the other layers,
there is no special handling for the case when the target
is the same size as the source.
Previously, the encoding of such spatial layers will use
completely uninitialized data, encoding complete garbage.
Instead force DownsamplePadding to make a copy if no scaling
is required, for the dependency layers. The base layer still
avoids a copy unless scaling of that layer is required.
Whether it actually makes sense to have lower spatial layers
the same size as the original one is a different question
though - currently the code allows it, and
EncodeDecodeTestAPI.SetOptionEncParamExt will try to use it.
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