gallium: add pipe_vertex_elements, lower 64-bit attribs in drivers, many cso_context changes

pipe_vertex_elements only contains an array of pipe_vertex_element and count, which must be the input to create_vertex_elements_state with no driver-specific modifications. This also changes void * to pipe_vertex_elements * everywhere. This is a massive commit.

64-bit vertex attribs need to be lowered in drivers because of how CSO caching of pipe_vertex_elements is going to work. This is also a very large commit.

The above changes are refactoring for cso_context and future work.

The other commits improve cso_context.