Minor bug fix for GMS2

Small bug fix after I got alerted to a problem with the source in GMS2 on twitter. In dungen_render_room(), the surface is now popped from the stack before it's freed, preventing potential issues with memory leaks and/or stack overflow. This is a one-line change, and if you want to fix this bug in your project, you should edit that script like this:

The new 1.1 version of the source file only contains this change and should otherwise be the same as the original version.