Fixes ArcticaProject/nx-libs#750

Backport of this commit:

commit e32cc0b4c85c78cd8743a6e1680dcc79054b57ce
Author: Adam Jackson <ajax@redhat.com>
Date:   Thu Apr 21 16:37:11 2011 -0400

    fb: Fix memcpy abuse

    The memcpy fast path implicitly assumes that the copy walks
    left-to-right.  That's not something memcpy guarantees, and newer glibc
    on some processors will indeed break that assumption.  Since we walk a
    line at a time, check the source and destination against the width of
    the blit to determine whether we can be sloppy enough to allow memcpy.
    (Having done this, we can remove the check for !reverse as well.)

    On an Intel Core i7-2630QM with an NVIDIA GeForce GTX 460M running in
    NoAccel, the broken code and various fixes for -copywinwin{10,100,500}
    gives (edited to fit in 80 columns):

    1: Disable the fastpath entirely
    2: Replace memcpy with memmove
    3: This fix
    4: The code before this fix

      1            2                 3                 4           Operation
      ------   ---------------   ---------------   ---------------   ------------
      258000   269000 (  1.04)   544000 (  2.11)   552000 (  2.14)   Copy 10x10
       21300    23000 (  1.08)    43700 (  2.05)    47100 (  2.21)   Copy 100x100
         960      962 (  1.00)     1990 (  2.09)     1990 (  2.07)   Copy 500x500

    So it's a modest performance hit, but correctness demands it, and it's
    probably worth keeping the 2x speedup from having the fast path in the
    first place.
Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Keith Packard <keithp@keithp.com>