Fix get_write_frame

Make variable that stores the write Pointer non-constant so python can
change the value. following code now makes a BlankClip red.

import vapoursynth as vs
core = vs.get_core()
clip = core.std.BlankClip(length=4)

def mod_frame(n, f):
print("Frame: {}".format(n))
fout = f.copy()
data = fout.get_write_frame(0)
for x,row in enumerate(data):
value = 255
for y, value in enumerate(row):
data[x,y] = 255
return fout

out=clip
out = core.std.ModifyFrame(clip, clip, selector=mod_frame)
print(out)
out.set_output()