draw: add map/unmap directives for swtnl driver interface

draw: add map/unmap directives for swtnl driver interface

Previously draw module asked for a pointer into (mapped) vertex data,
which it would incrementally fill and emit draw commands against.  This
was hard for the drivers to deal with, especially in the case where a
draw command would force a flush and thus an unmap of the vertex data.

With this change, the draw module explicitly maps & then unmaps vertex
data prior to emitting draw commands.