Support channels with subsampling

[tiagoshibata]

openexrpython currently breaks with channels using subsampling (eg. images with chroma subsampling, such as EXR's Y RY BY encoding).

How to reproduce

Run the following in openexr-images:

#!/bin/env python2
import OpenEXR
import pprint

filename = 'Chromaticities/Rec709_YC.exr'
assert OpenEXR.isOpenExrFile(filename)
exr_file = OpenEXR.InputFile(filename)
assert exr_file.isComplete()
pprint.pprint(exr_file.header())
exr_file.channel('RY')

Result:

{'channels': {'BY': HALF (2, 2), 'RY': HALF (2, 2), 'Y': HALF (1, 1)},
 'compression': PIZ_COMPRESSION,
 'dataWindow': (0, 0) - (609, 405),
 'displayWindow': (0, 0) - (609, 405),
 'lineOrder': INCREASING_Y,
 'owner': 'Copyright 2006 Industrial Light & Magic',
 'pixelAspectRatio': 1.0,
 'screenWindowCenter': (0.0, 0.0),
 'screenWindowWidth': 1.0}
Traceback (most recent call last):
  File "./crash.py", line 10, in <module>
    exr_file.channel('RY')
IOError: X and/or y subsampling factors of "RY" channel of input file "Chromaticities/Rec709_YC.exr" are not compatible with the frame buffer's subsampling factors.

Fix

Subsampling rate should be adjusted in this code instead of hardcoded to 1. Some more fixes are required for writing files.

I will do a pull request with the required fixes.