mesa/vbo: replace vbo_draw_method() with _mesa_set_drawing_arrays()

mesa/vbo: replace vbo_draw_method() with _mesa_set_drawing_arrays()

The arrays specified by ctx->Array._DrawArrays are used for all
vertex drawing via vbo_context::draw_prims().  Different arrays are
used for immediate mode, vertex arrays, display lists, etc.  Changing
from one to another requires updating derived/driver array state.

Before, we indirectly specifid the arrays with the gl_draw_method values.
Now we just directly specify the arrays instead.  This is simpler and
will allow a subsequent display list optimization.

In the future, it might make sense to get rid of ctx->Array._DrawArrays
entirely and just pass the arrays as another parameter to
vbo_context::draw_prims().

Reviewed-by: Mathias Fröhlich <mathias.froehlich@web.de>