Create missing directories before saving a PNG

[botoddly]

Fixes #614.

• SdlImageWriter.SavePng creates the missing parent directories of its path before writing the PNG. This covers the headless screenshot <path> command, which saves through it.
• A path that still cannot be written, such as one whose parent is a file, keeps throwing out of the frame loop.
• docs/headless.md says that missing directories are created, both for screenshot and for IImageWriter.

An app that registers its own IImageWriter does not get this behavior.

Verification

• Pixely.Tests filtered to SdlImageWriterTests and the input automation tests pass (95). A new test saves into a nested path that does not exist yet. Another checks that a parent that is a file throws IOException.
• Not tested: a full headless run, Windows and macOS.

[github-actions]

📦 Development package Pixely 0.0.112 is available.

Workflow run (attempt 1)

[github-actions]

✅ Development package validation passed on all platforms.

Workflow run (attempt 1)