Make implicit global usings opt-in

Author: JimBobSquarePants

ImageSharp.Web.sln

@@ -16,6 +16,7 @@ Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "_root", "_root", "{C317F1B1
 		Directory.Build.targets = Directory.Build.targets
 		LICENSE = LICENSE
 		README.md = README.md
+		SixLabors.ImageSharp.Web.props = SixLabors.ImageSharp.Web.props
 		shared-infrastructure\SixLabors.ruleset = shared-infrastructure\SixLabors.ruleset
 		shared-infrastructure\SixLabors.Tests.ruleset = shared-infrastructure\SixLabors.Tests.ruleset
 		shared-infrastructure\stylecop.json = shared-infrastructure\stylecop.json

SixLabors.ImageSharp.Web.props

@@ -2,7 +2,7 @@
 <Project>
 
   <!--Add common namespaces to implicit global usings if enabled.-->
-  <ItemGroup Condition="'$(ImplicitUsings)'=='enable' OR '$(ImplicitUsings)'=='true'">
+  <ItemGroup Condition="'$(UseImageSharp)'=='enable' OR '$(UseImageSharp)'=='true'">
     <Using Include="SixLabors.ImageSharp" />
     <Using Include="SixLabors.ImageSharp.Processing" />
     <Using Include="SixLabors.ImageSharp.Web" />