ImageMorph operations must have length 1

[radarhere]

https://pillow.readthedocs.io/en/stable/reference/ImageMorph.html

The default is to return the current pixel value, which is returned if no other match is found.

Operations:

• 4 - 4 way rotation
• N - Negate
• 1 - Dummy op for no other operation (an op must always be given)
• M - Mirroring

When parsing pattern strings, we currently allow operations of any length, even zero.

Pillow/src/PIL/ImageMorph.py

Line 153 in d560320

m = re.search(r"(\w*):?\s*\((.+?)\)\s*->\s*(\d)", p.replace("\n", ""))

This PR requires the operations to have length 1 instead.