Collada exporter: improve compatibility when exported path contains ':'.

Old code exported "c:/foo.jpg" path as "c%3a/foo.jpg".
Replacing : with %3a was probably legal, but it made paths unreadable for existing Max importers.

code/ColladaExporter.cpp

@@ -547,7 +547,7 @@ void ColladaExporter::WriteImageEntry( const Surface& pSurface, const std::strin
     std::stringstream imageUrlEncoded;
     for( std::string::const_iterator it = pSurface.texture.begin(); it != pSurface.texture.end(); ++it )
     {
-      if( isalnum_C( (unsigned char) *it) || *it == '_' || *it == '.' || *it == '/' || *it == '\\' )
+      if( isalnum_C( (unsigned char) *it) || *it == ':' || *it == '_' || *it == '.' || *it == '/' || *it == '\\' )
         imageUrlEncoded << *it;
       else
         imageUrlEncoded << '%' << std::hex << size_t( (unsigned char) *it) << std::dec;