0.2.6 - Beta 1

This is a beta to allow some people to test whether certain models they use work as expected. But anyone else should feel free to give it a try as well.

• New XPS render groups were implemented, so some models that didn't load or loaded only partially should work now.
• As a side-effect, users can select more shaders when trying out stuff.
• Some bugs in error-handling were fixed. Previously there were some situations where a model wouldn't load, and nothing else would load properly afterwards either. Now they get a proper error message, and other files continue to work as expected.

On the back-end, the whole way the shaders are set up has changed drastically to be much more flexible, so render results may differ in some situations. The most notable thing should be that the ambient colour is much more prominent (there were bugs that meant it got ignored in some situations before), so in a default scene, everything ends up looking a bit more grey than before.