plsmem: Set the memory area to be plotted

Set the memory area to be plotted (with the "mem" driver) as the dev member of the stream structure. Also set the number of pixels in the memory passed in plotmem, which is a block of memory maxy by maxx by 3 bytes long, say: 480 x 640 x 3 (Y, X, RGB)

This memory will have to be freed by the user!

maxx (PLINT, input) Size of memory area in the X coordinate. maxy (PLINT, input) Size of memory area in the Y coordinate. plotmem (void *, input) Pointer to the beginning of the user-supplied memory area.

Redacted form: plsmem(maxx, maxy, plotmem)

This function is not used in any examples.