Say when get_image and get_pixel can read the screen

[pvcraven]

Summary

Closes #1342 with a docstring note on arcade.get_image() and arcade.get_pixel(). The change is 7 lines in arcade/screenshot.py.

Call this at the end of on_draw, after drawing. Once a frame has been shown, such as in on_key_press, the screen's contents are undefined and may read as black.

Why

Saving a screenshot is already a one-liner, arcade.get_image().save("screenshot.png"). But the obvious beginner code calls it from a key press, after the frame has been shown. I tested it on Windows:

• Right after drawing, inside the frame: get_image() returns the drawing.
• After the frame is shown: it returns an all-black image.

Nothing in the docstrings warned about this. The get_image example's comment now says "At the end of on_draw".

The rest of #1342

• Coordinates: pushfoo's concern that get_image used a top-left origin is already fixed. It uses the bottom left and handles pixel ratio.
• Not added: a save_screenshot() helper and the screenshot manager, queueing and event extras. The one-liner covers saving, and nobody has asked for the extras since 2022.

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