shm: Don't try to put a whole bunch of clipped XY image contents.

sw and sh are smaller than w and h, and our temporary pixmap is only sw by sh, and we copy out an area of sw by sh. There's no need to go asking the driver to putimage w by h, and glamor acceleration tripped over this by making a temporary of the fbPutImage results of size w by h, without running it through the clip first.