Remove unnecessary system dependency

[cbarkr]

Resolves #135

Summary

As explained in the issue, rt-utils appears to run headlessly, in which case libGL is not necessary if the opencv-python-headless package is used instead of opencv-python.

This PR

• Changes opencv-python -> opencv-python-headless
• Removes libGL from the README
• Adds some sanity checks for find_mask_contours() and draw_line_upwards_from_point() - 2/3 functions using cv2 methods - from the image_helper module

Testing

I compared pytest output between the main branch and my own when run in a Docker container defined by the following Dockerfile:

FROM ubuntu

WORKDIR /app

# Explicitly *not* installing LibGL (libgl1 && libglib2.0-0)
RUN apt-get update && \
    apt-get install -y python3 python3-pip python3-venv && \
    rm -rf /var/lib/apt/lists/*

COPY . .

ENV PATH="/opt/venv/bin:$PATH"
RUN python3 -m venv /opt/venv && \
    pip3 install pytest && \
    pip3 install --no-cache-dir -r requirements.txt

CMD ["pytest"]

In main, the following error will occur:

rt_utils/__init__.py:1: in <module>
    from .rtstruct import RTStruct
rt_utils/rtstruct.py:5: in <module>
    from . import ds_helper, image_helper
rt_utils/ds_helper.py:2: in <module>
    from rt_utils.image_helper import get_contours_coords
rt_utils/image_helper.py:5: in <module>
    import cv2 as cv
/opt/venv/lib/python3.12/site-packages/cv2/__init__.py:181: in <module>
    bootstrap()
/opt/venv/lib/python3.12/site-packages/cv2/__init__.py:153: in bootstrap
    native_module = importlib.import_module("cv2")
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
E   ImportError: libGL.so.1: cannot open shared object file: No such file or directory