swrast: use memmove() instead of memcpy() in the SHIFT_ARRAY macro

swrast: use memmove() instead of memcpy() in the SHIFT_ARRAY macro

The src/dst arrays would overlap but dst was less than src so a simple
version of memcpy() would do the right thing.  But this isn't guaranteed
when memcpy() is optimized.

Fixes demos/copypix when the dest region was clipped by the left side of
the window.

Reviewed-by: Adam Jackson <ajax@redhat.com>