st/mesa: add support for layered framebuffers and consolidate code

st/mesa: add support for layered framebuffers and consolidate code

This is a subset of geometry shaders. It's all about setting first_layer and
last_layer correctly.

Also some code between st_render_texture and update_framebuffer_state is
consolidated. It doesn't use rtt_level and derives the level from dimensions
instead as the code in st_atom_framebuffer.c did.