Resize Callback fixed

Callbacks are executed in the main thread, but setting the viewport
should happen in the render thread.
Purgelist fixed

Fixed segfault when purging unused patches.
Comments on generation

Patches and their generation will be slightly restructured, they are now
in either of three states, indicating the data associated with them.
While the checks whether a patch will be rendered and in which order
they will be rendered will remain largely unchanged, their deletion and
generation will undergo some restructuring.