Allow Build with Visual Studio For Mac

[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 matches 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

Visual Studio for Mac has a pretty sever limitation when building multitargeting projects.

The current implementation allows multi-target frameworks projects to be opened and compiled but treats them as though they have a single target framework.

https://bugzilla.xamarin.com/show_bug.cgi?id=55508#c5

This PR reorders the target frameworks so that netcoreapp2.1 is the first target in all relevant projects and adds the target to ImageSharp.Drawing (we'll probably need it for future perf work anyway).

[codecov]

Codecov Report

Merging #833 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #833     +/-   ##
=========================================
- Coverage   88.84%   88.75%   -0.1%     
=========================================
  Files        1015     1015             
  Lines       44167    44167             
  Branches     3181     3181             
=========================================
- Hits        39241    39201     -40     
- Misses       4201     4206      +5     
- Partials      725      760     +35

Impacted Files	Coverage Δ
...xelFormats/PixelOperations{TPixel}.PixelBenders.cs	89.34% <0%> (-9.84%)	⬇️
...Processing/Processors/Transforms/TransformUtils.cs	88.29% <0%> (-5.32%)	⬇️
src/ImageSharp/Common/Helpers/ImageMaths.cs	82.05% <0%> (-5.13%)	⬇️
.../Processing/Processors/Transforms/CropProcessor.cs	96% <0%> (-4%)	⬇️
...ocessing/Processors/Dithering/ErrorDiffuserBase.cs	93.75% <0%> (-3.13%)	⬇️
...rs/Normalization/HistogramEqualizationProcessor.cs	97.5% <0%> (-2.5%)	⬇️
...eSharp.Tests/IO/DoubleBufferedStreamReaderTests.cs	97.61% <0%> (-2.39%)	⬇️
.../ImageSharp.Tests/TestUtilities/TestEnvironment.cs	79.16% <0%> (-2.09%)	⬇️
...c/ImageSharp/Formats/Png/Zlib/ZlibDeflateStream.cs	61.4% <0%> (-1.76%)	⬇️
...TestUtilities/Attributes/ImageDataAttributeBase.cs	80% <0%> (-1.67%)	⬇️
... and 12 more

---

Continue to review full report at Codecov.

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #833 into master will decrease coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #833     +/-   ##
=========================================
- Coverage   88.84%   88.75%   -0.1%     
=========================================
  Files        1015     1015             
  Lines       44167    44167             
  Branches     3181     3181             
=========================================
- Hits        39241    39201     -40     
- Misses       4201     4206      +5     
- Partials      725      760     +35

Impacted Files	Coverage Δ
...xelFormats/PixelOperations{TPixel}.PixelBenders.cs	89.34% <0%> (-9.84%)	⬇️
...Processing/Processors/Transforms/TransformUtils.cs	88.29% <0%> (-5.32%)	⬇️
src/ImageSharp/Common/Helpers/ImageMaths.cs	82.05% <0%> (-5.13%)	⬇️
.../Processing/Processors/Transforms/CropProcessor.cs	96% <0%> (-4%)	⬇️
...ocessing/Processors/Dithering/ErrorDiffuserBase.cs	93.75% <0%> (-3.13%)	⬇️
...rs/Normalization/HistogramEqualizationProcessor.cs	97.5% <0%> (-2.5%)	⬇️
...eSharp.Tests/IO/DoubleBufferedStreamReaderTests.cs	97.61% <0%> (-2.39%)	⬇️
.../ImageSharp.Tests/TestUtilities/TestEnvironment.cs	79.16% <0%> (-2.09%)	⬇️
...c/ImageSharp/Formats/Png/Zlib/ZlibDeflateStream.cs	61.4% <0%> (-1.76%)	⬇️
...TestUtilities/Attributes/ImageDataAttributeBase.cs	80% <0%> (-1.67%)	⬇️
... and 12 more

---

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 892bba1...ac5d945. Read the comment docs.