Include System.Drawing primitives in netstandard about standard HOT 6 CLOSED

@akoeplinger @marek-safar Please have a look at e2425ce where I needed to add Xamarin.iOS.dll and OpenTK-1.0.dll to the reference set to correctly generate the facade for the drawing primitives. You guys may want to consider adding a System.Drawing.dll, even if it is only a facade so that you can share the same System.Drawing facade we are generating for netstandard compat.

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The change looks ok but the newly added assemblies have wrong version number and public key. We don't version them. They both have 0.0.0.0 version and custom public key. @akoeplinger could you PR a fix for that?

About adding System.Drawing facade. I think we tried that some time ago but it was a breaking change but would have to dig out for exact details but I think it was due to user's provided System.Drawing.dll (which is quite common) and some resolution conflicts caused by that.

Another option can be to re-use existing System.Drawing.Primitives.dll facade if you want.

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Thanks for the information. This does mean we are going to need a unique netstandard facade for you guys as the one we are building for net461 will not just work for you, but that shouldn't be a big deal either way.

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Yes, this will get more complicated. The netstandard.dll needs to be different for every target platform (android, ios, ios-watch, ios-tv, mac, etc) because we have different set of assemblies there. Something like

ios - has OpenTK + Xamarin.iOS
android - has System.Drawing
ios-watch - has none of that
mac - has OpenTK + Xamarin.Mac

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OK we will need to figure out how we want to set that up for you to be able to sub-module dotnet/standard and build unique facades for each of those targets. However it shouldn't be too terrible.

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I added the other platforms in #96 with correct version numbers + keys.

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