Smudge enhancement: second version (may require a final polish)

1.Now, smudge displaces pixels. (in bitmap layer)
2. Slow drag produces produces small  displacement while quick drag
produces larger displacement.
3. Smoother results by setting feather to maximum value.

Remark: The added function drawTexturedBrush (similar to drawBrush) can
be used to easily create a "cloner tool" or to add static textures to
the "brush".
Merge pull request #189 from eVoLuTiVeMEDIA/master

Smudge enhancement.
Merge branch 'master' of https://github.com/chchwy/pencil2d