Added function 'gdImageGetInterpolationMethod()'.

This retrieves the interpolation method set via
gdImageSetInterpolationMethod().  It is a function instead of a macro
for the benefit of VMs that need a function pointer to call.
Started importing the old manual into Naturaldocs comments.

This changeset imports documention for gdImage to gdImagePng*() into
Natural Docs.  Documention is based on the text of the original
manual, version 2.0.36 but adapted to better suit the format.
Subsequent changesets will introduce docs for subsequent manual
entries.

naturaldocs, when present, is invoked by bootstrap.sh.  The completed
manual will be in docs/naturaldocs/html/index.html.  It can also be
explicitly invoked by running docs/naturaldocs/run_docs.sh.

This change also removed docs/naturaldocs/project/Menu.txt, since it
currently contains no non-generated content and is prone to introduce
noise into the changeset.