fltk backend fix & mouse wheel scroll factor

[John W. Eaton]

On 27-Sep-2009, Shai Ayal wrote:

| The attached changset fixes a bug which did not show the zoom box.
| Also added a function to enable the user to control the zoom factor
| when using mouse wheel zoom.
| 
| I think that with this changeset, the fltk_backend is quite usable as
| a viewer to the gl-renderer.

I applied it.  Zooming seems to work well for me now with either the
scroll wheel or the click and drag method.

| I am open to feature requests.

Just about zooming/rotating issues, or other things?  :-)

If we just had images working, then the fltk backend might be able to
run all the demos...

jwe