Use Rgb24 for Async Jpeg Decode

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Non-generic async jpeg decoding was still using Rgba32 by default.

[codecov]

Codecov Report

Merging #1859 (f415e37) into master (4d0bc4a) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1859   +/-   ##
======================================
  Coverage      87%     87%           
======================================
  Files         935     935           
  Lines       48751   48751           
  Branches     6081    6081           
======================================
+ Hits        42585   42590    +5     
+ Misses       5155    5153    -2     
+ Partials     1011    1008    -3     

Flag	Coverage Δ
unittests	87% <100%> (+<1%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Jpeg/JpegDecoder.cs	95% <100%> (ø)
...rc/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs	97% <0%> (+<1%)	⬆️
...ageSharp/Formats/Webp/Lossless/PredictorEncoder.cs	87% <0%> (+<1%)	⬆️
...ImageSharp/Formats/Webp/Lossless/Vp8LBitEntropy.cs	100% <0%> (+1%)	⬆️
...mageSharp/Formats/Webp/Lossless/NearLosslessEnc.cs	100% <0%> (+3%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4d0bc4a...f415e37. Read the comment docs.

[antonfirsov]

Good catch, thanks! If you have a bit of more time an equivalent async test might be useful:

ImageSharp/tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.cs

Lines 88 to 94 in d81c511

	[Fact]
	public void Decode_NonGeneric_CreatesRgb24Image()
	{
	string file = Path.Combine(TestEnvironment.InputImagesDirectoryFullPath, TestImages.Jpeg.Baseline.Jpeg420Small);
	using var image = Image.Load(file);
	Assert.IsType<Image<Rgb24>>(image);
	}

[antonfirsov]

👍