[python api] openslide.read_region broke when svs too large

[fengzhi09]

Context

Issue type (bug report or feature request): bug report
Operating system (e.g. Fedora 24, Mac OS 10.11, Windows 10): Windows 10
Platform (e.g. 64-bit x86, 32-bit ARM): 64-bit x86
OpenSlide version: 1.1.1
Slide format (e.g. SVS, NDPI, MRXS): svs

Details

call like this:
openslide.open_slide(svs_file)
level = 1
dimension=slide.level_dimensions[level]
// It broken many times.
// Among all broken dimensions of all svs files, min is (25895,21005) max is (89639,51222)
slide.read_region((0, 0), level, dimension)

stack_trace:
File "D:\projects\cancer_ml_project\tools\SVS2PNGConverter.py", line 31, in convert_for_file
imsave(out_folder + "/" + png_file, np.array(slide.read_region((0, 0), level, dimension)))
File "D:\projects\cancer_ml_project\venv\lib\site-packages\openslide_init_.py", line 223, in read_region
level, size[0], size[1])
File "D:\projects\cancer_ml_project\venv\lib\site-packages\openslide\lowlevel.py", line 258, in read_region
buf = (w * h * c_uint32)()
OverflowError: The 'length' attribute is too large

[jxu]

read_region is for reading relatively small regions that will fit into memory (probably image area < 2^32)
It's usually not possible to read the whole region into memory anyway. That's the point of OpenSlide

[jxu]

Maybe duplicate of openslide/openslide#73