Target .NET Standard where possible, restore Virtu TFM

fixes 5a0992c2b

ExternalCoreProjects/Virtu/Virtu.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 	<Import Project="$(ProjectDir)../../Common.props" />
 	<PropertyGroup>
+		<TargetFrameworks>netstandard2.0;netcoreapp3.1</TargetFrameworks>
 	</PropertyGroup>
 	<ItemGroup>
 		<Reference Include="Microsoft.CSharp" />

ExternalProjects/FlatBuffers/FlatBuffers.Core.csproj

@@ -1,5 +1,8 @@
 <Project Sdk="Microsoft.NET.Sdk">
 	<Import Project="../../src/MainSlnCommon.props" />
+	<PropertyGroup>
+		<TargetFramework>netstandard2.0</TargetFramework>
+	</PropertyGroup>
 
     <ItemGroup>
       <Compile Remove="Properties\**" />

src/BizHawk.Bizware.BizwareGL/BizHawk.Bizware.BizwareGL.csproj

@@ -2,6 +2,7 @@
 	<Import Project="../MainSlnCommon.props" />
 	<PropertyGroup>
 		<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+		<TargetFramework>netstandard2.0</TargetFramework>
 	</PropertyGroup>
 	<ItemGroup>
 		<PackageReference Include="Microsoft.CSharp" Version="4.7.0" /><!-- for (extra?) dynamic type support -->

src/BizHawk.Client.Common/BizHawk.Client.Common.csproj

@@ -2,11 +2,11 @@
 	<Import Project="../MainSlnCommon.props" />
 	<PropertyGroup>
 		<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+		<TargetFramework>netstandard2.0</TargetFramework><!-- some NuGet packages aren't for Standard, and are only loaded at runtime because EmuHawk is still Framework. More work will be required to move to Core. -->
 	</PropertyGroup>
 	<ItemGroup>
 		<Reference Include="System.Data.SQLite, Version=1.0.105.2, Culture=neutral, PublicKeyToken=db937bc2d44ff139, processorArchitecture=AMD64" SpecificVersion="False" HintPath="$(ProjectDir)../../References/x64/SQLite/System.Data.SQLite.dll" />
 		<Reference Include="NLua" HintPath="$(ProjectDir)../../output/dll/nlua/NLua.dll" Private="False" />
-		<Reference Include="System.IO.Compression" />
 		<PackageReference Include="SharpCompress" Version="0.24.0" />
 		<PackageReference Include="SharpZipLib" Version="1.1.0" />
 		<ProjectReference Include="$(ProjectDir)../BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj" />

src/BizHawk.Common/BizHawk.Common.csproj

@@ -3,8 +3,7 @@
 	<Import Project="../../CommonNullable.props" />
 	<PropertyGroup>
 		<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
-	</PropertyGroup>
+		<TargetFramework>netstandard2.0</TargetFramework>
-	<PropertyGroup>
 	</PropertyGroup>
 	<ItemGroup>
 		<PackageReference Include="System.ComponentModel.Annotations" Version="4.7.0" />

src/BizHawk.Emulation.Common/BizHawk.Emulation.Common.csproj

@@ -2,6 +2,7 @@
 	<Import Project="../MainSlnCommon.props" />
 	<PropertyGroup>
 		<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+		<TargetFramework>netstandard2.0</TargetFramework>
 	</PropertyGroup>
 	<ItemGroup>
 		<PackageReference Include="Newtonsoft.Json" Version="12.0.3" />

src/BizHawk.Emulation.Cores/BizHawk.Emulation.Cores.csproj

@@ -2,6 +2,7 @@
 	<Import Project="../MainSlnCommon.props" />
 	<PropertyGroup>
 		<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+		<TargetFramework>netstandard2.0</TargetFramework>
 	</PropertyGroup>
 	<ItemGroup>
 		<PackageReference Include="ELFSharp" Version="2.10.0" />

src/BizHawk.Emulation.DiscSystem/BizHawk.Emulation.DiscSystem.csproj

@@ -2,6 +2,7 @@
 	<Import Project="../MainSlnCommon.props" />
 	<PropertyGroup>
 		<AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+		<TargetFramework>netstandard2.0</TargetFramework>
 	</PropertyGroup>
 	<ItemGroup>
 		<ProjectReference Include="$(ProjectDir)../BizHawk.Common/BizHawk.Common.csproj" />

src/BizHawk.Tests/BizHawk.Tests.csproj

@@ -4,6 +4,7 @@
 	<PropertyGroup>
 		<AppendTargetFrameworkToOutputPath>false</AppendTargetFrameworkToOutputPath>
 		<OutputPath>$(ProjectDir)../../test_output</OutputPath>
+		<TargetFramework>netcoreapp3.1</TargetFramework>
 	</PropertyGroup>
 	<ItemGroup>
 		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.6.1" PrivateAssets="all" />

src/MainSlnCommon.props

@@ -6,9 +6,6 @@
 		<NoWarn>NU1701</NoWarn>
 		<OutputPath>$(ProjectDir)../../output/dll</OutputPath>
 	</PropertyGroup>
-	<PropertyGroup>
-		<TargetFrameworks>net48</TargetFrameworks>
-	</PropertyGroup>
 	<ItemGroup>
 		<AdditionalFiles Include="$(ProjectDir)../../.stylecop.json" />
 	</ItemGroup>