imagemagick-render-type fixes

imagemagick-render-type fixes

* src/image.c (imagemagick-render-type): Change it from a lisp object
to an integer.  Move the doc here from the lisp manual.
Treat all values not equal to 0 the same.

* doc/lispref/display.texi (ImageMagick Images):
Move most details of imagemagick-render-type to the variable's doc.

* lisp/cus-start.el (imagemagick-render-type): Add it.