Fixed LoD painting for Imagepipe and Text Brushes

Now every KisPainInformation has it's own source of random numbers,
which must be used by all the paint ops and brushes. The sharing and
forking of the source is performed automatically by the c-tor of the
KisPaintInformation.

NOTE: all this 'painting source' adventure is based on an assumption that
      painting on a scaled down plan scales linearly, and therefore the
      number of calls to the random source will be strictly the same.
      This is true for most of the brushes, but some settings can break
      this assumption, e.g. if we have Auto Spacing (which is non-linear)
      or if we add some weird sensor to Spacing option.

CC:kimageshop@kde.org