gdal2xyz: Change -srcwin parameter type to integer.

[eliepa]

PR fixes -srcwin parameter type

Window rectangle corners shall be specified as integers.

[jratike80]

Is it a problem if it is float? If I read right https://github.com/OSGeo/gdal/blob/master/apps/gdal_translate_lib.cpp#L291 gdal_translate is using doubles. What if the windows is computed by code that can return something else than integers due to re-projection or something?

[eliepa]

Yes, there is a problem with float:
x_off, y_off, x_size, y_size = srcwin
and later:
for x_i in range(0, x_size, x_skip):
which causes an error.

[coveralls]

coverage: 68.946% (-0.003%) from 68.949%
when pulling ac3724f on eliepa:fix_srcwin
into cf119e5 on OSGeo:master.