fltk backend fix & mouse wheel scroll factor

[Shai Ayal]

Hi

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 am open to feature requests.

Shai

fltk_fixes.chngeset
Description: Binary data