Do not use "use built-in mapper WIN32 only"

[cgohlke]

Fixes the issue reported at https://stackoverflow.com/questions/59485047/importing-large-multilayer-tiff-as-numpy-array

The Win32 only mapper is 32-bit only and cannot access more than 2 GB. This code is ancient and can probably be replaced by Python's memmap.

import numpy as np
from PIL import Image

# generate test image
fname = r"test.tif"
img = np.random.randint(2**16, size=(257, 2048, 2048), dtype=np.uint16)

# save test image
im_list = [Image.fromarray(img[ii].astype('uint16')) for ii in range(img.shape[0])]
im_list[0].save(fname, save_all=True, append_images=im_list[1:])

# load image
im_loaded = Image.open(fname)

# this succceeds
im_loaded.seek(255)
arr255 = np.asarray(im_loaded, dtype=np.uint16)

# this fails
im_loaded.seek(256)
im_loaded.show()
arr256 = np.asarray(im_loaded, dtype=np.uint16)

File "pil2gbtiff.py", line 21, in <module>
  im_loaded.show()
File "X:\Python36\Lib\site-packages\PIL\Image.py", line 2128, in show
  _show(self, title=title, command=command)
File "X:\Python36\Lib\site-packages\PIL\Image.py", line 3040, in _show
  _showxv(image, **options)
File "X:\Python36\Lib\site-packages\PIL\Image.py", line 3046, in _showxv
  ImageShow.show(image, title, **options)
File "X:\Python36\Lib\site-packages\PIL\ImageShow.py", line 54, in show
  if viewer.show(image, title=title, **options):
File "X:\Python36\Lib\site-packages\PIL\ImageShow.py", line 72, in show
  image = image.convert(base)
File "X:\Python36\Lib\site-packages\PIL\Image.py", line 930, in convert
  self.load()
File "X:\Python36\Lib\site-packages\PIL\TiffImagePlugin.py", line 1098, in load
  return super(TiffImageFile, self).load()
File "X:\Python36\Lib\site-packages\PIL\ImageFile.py", line 186, in load
  self.map.seek(offset)

builtins.OverflowError: Python int too large to convert to C long

Changes proposed in this pull request:

• use Python's memory mapping instead.

[homm]

1. Need a test that fails with the previous implementation.
2. All related code in _imaging.c and map.c could be removed?

[radarhere]

I've recreated this as #5224, since this needs to be rebased and the forked repository for this PR has disappeared.