Jpeg RGB & grayscale encoders can be accelerated with intrinsics

[br3aker]

Current rgb and grayscale encoders use scalar code to cast data from byte to float:

ImageSharp/src/ImageSharp/Formats/Jpeg/Components/Encoder/LuminanceForwardConverter{TPixel}.cs

Lines 52 to 56 in 2b85b55

	for (int i = 0; i < Block8x8F.Size; i++)
	{
	ref L8 c = ref Unsafe.Add(ref l8Start, i);
	yBlock[i] = c.PackedValue;
	}

ImageSharp/src/ImageSharp/Formats/Jpeg/Components/Encoder/RgbForwardConverter{TPixel}.cs

Lines 104 to 111 in 2b85b55

	for (int i = 0; i < Block8x8F.Size; i++)
	{
	Rgb24 c = Unsafe.Add(ref rgbStart, (nint)(uint)i);
	redBlock[i] = c.R;
	greenBlock[i] = c.G;
	blueBlock[i] = c.B;
	}

Grayscale code can be simd'ified via avx VPMOVZXBD aka Avx2.ConvertToVector256Int32 & VCVTDQ2PS aka Avx.ConvertToVector256Single commands. Rgb encoder is a bit trickier as it would involve shuffles - needs further benchmarking.

Anyway, this is fairly straightforward so it's a 'good first issue' issue.