winex11: Introduce get_xrender_picture for retrieving a Picture from a physDev and allocating a Picture when needed.

This was introduced by abb11f30, but I don't
see how it could ever do something useful. Perhaps that's a lack of
imagination on my part, but the only situation in which device->stateBlock
should be NULL is while creating the initial stateblock, and in that case we
just returned in the line above. Setting the stream frequency of all streams
to 0x01010101 doesn't make sense either.