mesa: move gl_client_array*[] from vbo_draw_func into gl_context

mesa: move gl_client_array*[] from vbo_draw_func into gl_context

In the future we'd like to treat vertex arrays as a state and
not as a parameter to the draw function. This is the first step
towards that goal. Part of the goal is to avoid array re-validation
for every draw call.

This commit adds:
const struct gl_client_array **gl_context::Array::_DrawArrays.

The pointer is changed in:
* vbo_draw_method
* vbo_rebase_prims - unused by gallium
* vbo_split_prims - unused by gallium
* st_RasterPos

Reviewed-by: Brian Paul <brianp@vmware.com>