mlt_properties.c: make arithmetic processor use floating point instead of integer...

mlt_properties.c: make arithmetic processor use floating point instead of integer so that '/' is meaningful. I am not totally certain of the consequences of this change because I am not aware of where the feature is used. However, I am using it to specify the aspect ratio of certain things like bitmap graphics that were not designed for square pixels. And being able to specify a fraction allows for accurate detection of equivalent aspect ratios between different sources, particularly compositing.

git-svn-id: https://mlt.svn.sourceforge.net/svnroot/mlt/trunk/mlt@1130 d19143bc-622f-0410-bfdd-b5b2a6649095