loader: Merge extensions and layers

loader: Merge extensions and layers

Update loader so that it extensions and layers work in same
basic fashion. That is, application can query for extension/layer
support by calling GetExtensionSupport. The loader will then query
the available drivers AND layers for that support. Likewise,
when the application does CreateDevice the list of "extensions"
can be both layers and extensions and the loader will figure out
the right thing to do.

Conflicts:
	loader/loader.c
	xgl-generate.py