Adapt to the dynamic texture list in QRhiShaderResourceBinding

The texture and sampler list moved out of the union in
QRhiShaderResourceBinding::Data into a QLA, so the binding
list helper that fills the data directly has to resize the list
instead of setting a count.

Change-Id: I03161ca4a9a796e7b62c1f5529bcee5bf740d719
Reviewed-by: Andy Nichols <andy.nichols@qt.io>