Access builtin resources through a global instance

Explicitly passing the Resources reference causes problems when some
classes in a hierarchy need it and others don't.  It's also awkward in
RenderPass::set_material and PostProcessor templates.  It doesn't even
matter which instance is passed in since the data is builtin anyway.

The first created Resources instance is now registered as the one to use
for builtin data, unless suppressed by a constructor parameter.