Do not convert images unnecessarily in ImageEnhance

[radarhere]

If an attempt is made to convert an image to its current mode (without a matrix), a copy is simply returned.

However, copy() still consumes resources, and it would be ideal to avoid it where possible. In ImageEnhance, there are a few such instances.

What if the image already matches self.intermediate_mode in Color?

Pillow/src/PIL/ImageEnhance.py

Line 58 in c6b08ef

self.degenerate = image.convert(self.intermediate_mode).convert(image.mode)

What if the image is already L mode in Enhance?

Pillow/src/PIL/ImageEnhance.py

Lines 69 to 72 in c6b08ef

	def __init__(self, image: Image.Image) -> None:
	self.image = image
	mean = int(ImageStat.Stat(image.convert("L")).mean[0] + 0.5)
	self.degenerate = Image.new("L", image.size, mean).convert(image.mode)

This PR skips calling convert() for those scenarios.