mesa: Keep track of the last looked-up VAO

mesa: Keep track of the last looked-up VAO

This saves the cost of repeated hash table lookups when the same
vertex array object is referenced in a sequence of calls such as:

    glVertexArrayAttribFormat(vao, ...);
    glVertexArrayAttribBinding(vao, ...);
    glEnableVertexArrayAttrib(vao, ...);
    ...

Note that VAO's are container objects that are not shared between
contexts.

Reviewed-by: Laura Ekstrand <laura@jlekstrand.net>