Improved reportwriter's ability to locate images.

This fixes 2 problems:

1) First try self.Application.HomeDirectory if available;
   only if that fails try self.HomeDirectory which is defined
   as the location of the rfxml file.

2) If the end-result is a path that has both windows-style and
   unix-style separators, make them consistent with whatever
   platform we are running on.

I had relative paths to image files coming from a database
field, and the paths were stored in the database using unix
slash separators, but running the app on Windows resulted in
paths like "c:\imagelibrary\frames/h_111.png" which confused
things and resulted in an expr error on the pdf instead of the
image.