Composite: don't write to null pixels

[sjoerd108]

What's Changing and Why

This fixes a bug with the composite function which would write garbage data to a pixel at 0, 0 when the target pixel doesn't exist, which can happen when:

• The source image is bigger than the destination image
• Using negative x, y offsets

This is because getPixelIndex() returns -1 when using EDGE_CROP which didn't get checked for. So when writing to the target the red channel maps to -1 in the destination buffer, which gets discarded silently (Buffers surprisingly don't throw when you do this) and then the GBA parts of the source get written to the RGB channels on the target.

You can actually see it in the testing image I fixed for this PR: The old "correct" one has a random, off color pixel in the top left, the new one doesn't.

What else might be affected

Should not affect anything.

Tasks

• Add tests
• Update Documentation
• Update jimp.d.ts
• Add SemVer Label

[github-actions]

🚀 PR was released in v0.22.8 🚀