pil2cv: allow (and drop) alpha channels on input imgs

[bertsky]

The error I got by calling cvtColor as it was:

OpenCV(4.8.0) /io/opencv/modules/imgproc/src/color.simd_helpers.hpp:92: error: (-2:Unspecified error) in function 'cv::impl::{anonymous}::CvtHelper<VScn, VDcn, VDepth, sizePolicy>::CvtHelper(cv::InputArray, cv::OutputArray, int) [with VScn = cv::impl::{anonymous}::Set<1>; VDcn = cv::impl::{anonymous}::Set<3, 4>; VDepth = cv::impl::{anonymous}::Set<0, 2, 5>; cv::impl::{anonymous}::SizePolicy sizePolicy = cv::impl::<unnamed>::NONE; cv::InputArray = const cv::_InputArray&; cv::OutputArray = const cv::_OutputArray&]'
> Invalid number of channels in input image:
>     'VScn::contains(scn)'
> where
>     'scn' is 2

Removing the alpha channel in advance avoids the problem.

But perhaps we should not naively drop the channel, but use it (somehow) to mask the image? (I.e. for segmentation: no layout analysis in fully transparent zones, for binarization: pure background)

[vahidrezanezhad]

@bertsky @kba @cneud Even without any changes, including this PR, Eynollah (which I tested using my latest PR ) works correctly on this image with an alpha channel
. It seems this issue happens with other images with alpha channel. can you provide me images with alpha channel that this issue happens @bertsky ?

[kba]

If we decide to move ahead with this, we should backport that to OCR-D/core whence I took it originally.