Downgrade `System.Drawing.Common` to 6.0.0 and enable Unix impl.

fixes 65ffa3fc2
testroms run now, CBB checking GambatteSuite runs but I'm guessing it does too

ExternalProjects/PcxFileTypePlugin.HawkQuantizer/PcxFileTypePlugin.HawkQuantizer.csproj

@@ -8,6 +8,6 @@
 		<Nullable>disable</Nullable>
 	</PropertyGroup>
 	<ItemGroup>
-		<PackageReference Include="System.Drawing.Common" Version="7.0.0" />
+		<PackageReference Include="System.Drawing.Common" Version="6.0.0" />
 	</ItemGroup>
 </Project>

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

@@ -9,7 +9,7 @@
 	</PropertyGroup>
 	<ItemGroup>
 		<PackageReference Include="Cyotek.Drawing.BitmapFont" Version="2.0.4" />
-		<PackageReference Include="System.Drawing.Common" Version="7.0.0" />
+		<PackageReference Include="System.Drawing.Common" Version="6.0.0" />
 		<ProjectReference Include="$(ProjectDir)../BizHawk.Common/BizHawk.Common.csproj" />
 	</ItemGroup>
 	<ItemGroup>

src/BizHawk.Tests.Testroms.GB.GambatteSuite/BizHawk.Tests.Testroms.GB.GambatteSuite.csproj

@@ -10,5 +10,6 @@
 		<PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.5.0" PrivateAssets="all" />
 		<ProjectReference Include="$(ProjectDir)../BizHawk.Tests.Testroms.GB/BizHawk.Tests.Testroms.GB.csproj" />
 		<EmbeddedResource Include="res/**/*" />
+		<RuntimeHostConfigurationOption Include="System.Drawing.EnableUnixSupport" Value="true" />
 	</ItemGroup>
 </Project>

src/BizHawk.Tests.Testroms.GB/BizHawk.Tests.Testroms.GB.csproj

@@ -25,5 +25,6 @@
 		<ProjectReference Include="$(ProjectDir)../BizHawk.Client.Common/BizHawk.Client.Common.csproj" />
 		<EmbeddedResource Include="res/**/*" />
 		<Content Include="$(ProjectDir)../../Assets/gamedb/**/*" LinkBase="gamedb" CopyToOutputDirectory="PreserveNewest" />
+		<RuntimeHostConfigurationOption Include="System.Drawing.EnableUnixSupport" Value="true" />
 	</ItemGroup>
 </Project>