Enable "strict" compiler flag and fix noncompliance

couldn't find official docs for this, but I got it from:
https://www.meziantou.net/csharp-compiler-strict-mode.htm
and it definitely does SOMETHING

Common.props

@@ -4,6 +4,7 @@
 		<ContinuousIntegrationBuild Condition=" '$(GITLAB_CI)' != '' Or '$(APPVEYOR)' != '' ">true</ContinuousIntegrationBuild>
 		<EnableNETAnalyzers>true</EnableNETAnalyzers>
 		<ErrorReport>prompt</ErrorReport>
+		<Features>strict</Features>
 		<IsTargetingNetFramework>$(TargetFramework.StartsWith("net4"))</IsTargetingNetFramework>
 		<LangVersion>9.0</LangVersion>
 		<Nullable>enable</Nullable>

src/BizHawk.Client.Common/Api/Classes/GuiApi.cs

@@ -67,7 +67,8 @@ namespace BizHawk.Client.Common
 			return g;
 		}
 
-		public void ToggleCompositingMode() => _compositingMode = 1 - _compositingMode;
+		public void ToggleCompositingMode()
+			=> _compositingMode = (CompositingMode) (1 - (int) _compositingMode); // enum has two members, 0 and 1
 
 		public ImageAttributes GetAttributes() => _attributes;