allow eval expressions for icon-image and fill-image

If those tags contained an eval expression those would erroneously be taken as
a filename, leading to a parse error. Simply check if the expression is an eval
before calling the filename handling.

This does not make it really work, but it at least not crash.

mapcss_converter.py

@@ -216,7 +216,7 @@ def style_statement_as_js(self, subpart):
             value_tags.add(val)
         return "            s_%s[%s] = MapCSS.e_localize(tags, %s);" % (subpart, k, val)
     else:
-        if self.key in ('icon-image', 'fill-image'):
+        if not isinstance(self.value, ast.Eval) and self.key in ('icon-image', 'fill-image'):
             images.add(self.value)
         return "            s_%s[%s] = %s;" % (subpart, k, val)