Non-generic Image.Load should decode Jpeg into Image<Rgb24> #1410

Prerequsite: #1354

• Top gain: reduced memory footprint
• There will be a noticeable LoadResizeSave speed regression on 2.1 and Framework (no SIMD for shuffles), but I hope the ROTR / ROTL path will keep it in an acceptable range.
• On 3.1 and .NET 5 the LoadResizeSave speed regression should be negligible thanks to the SIMD path in 3 <==> 4 Channel Shuffling with Hardware Intrinsics #1409
• Introduce PixelTypeInfo.HasAlpha #1396 combined with omitting premultipication for Rgb24 will cancel out these regressions, bringing noticeable speedup on 3.1 and .NET 5. We may want to consider milestoning that issue to 1.1.0