Allow show to write the screenshot to a file-like object - #2136

Fixes #1760

show(..., screenshot=...) now also takes a binary file-like object such as io.BytesIO. In that case vtkPNGWriter writes to memory (WriteToMemoryOn) and the PNG bytes are written to the object, so docs or notebooks can get the image without going through a temporary file:

from io import BytesIO

buf = BytesIO()
show(b, screenshot=buf, interact=False)
png = buf.getvalue()

A file name works as before. The check is hasattr(screenshot, "write") rather than isinstance(screenshot, str) because VTK also takes a pathlib.Path in SetFileName, and that should keep working. The example above is added to the Screenshots section of doc/vis.rst.

The new test_screenshot_to_memory uses the patch_vtk fixture, but feeds the real vtkPNGWriter a 10x5 image from vtkImageCanvasSource2D in place of the window, and checks the PNG signature and the size in the IHDR chunk. On master it fails with TypeError: SetFileName argument 1.

Checked locally (Windows 10, Python 3.12, pip install with cadquery-ocp 8.0.1.1, VTK 9.6.2):

• off-screen render of a box at 320x240: the bytes in the BytesIO are identical to the file written with screenshot="img.png"
• pytest -n 8: 904 passed, 3 skipped (master: 903 passed, 3 skipped), no new warnings
• black . --diff --check (cq fork): nothing to reformat
• mypy cadquery: same errors as master; here they are almost all import-untyped for OCP, since the pip package has no stubs
• docs build with Sphinx 9.1.0: no new warnings, and the new example shows up under Screenshots

Happy to adjust anything if needed, e.g. if you'd rather have show return the bytes.

AI disclosure: this change was written with the help of Claude Code (Claude Opus 5.5).