Reworked Screenshot saving.

Now OGL doesn't rely on WX for PNG saving.
FlipImageData supports (pixel data len > 3) now.
TextureToPng is now in ImageWrite.cpp/h
Video Common depends on zlib and png.
D3D no longer depends on zlib and png.

Source/Core/VideoBackends/D3D/D3D.vcxproj

@@ -93,15 +93,9 @@
     <ClInclude Include="Src\XFBEncoder.h" />
   </ItemGroup>
   <ItemGroup>
-    <ProjectReference Include="..\..\..\..\Externals\libpng\png\png.vcxproj">
-      <Project>{4c9f135b-a85e-430c-bad4-4c67ef5fc12c}</Project>
-    </ProjectReference>
     <ProjectReference Include="..\..\..\..\Externals\wxWidgets3\build\msw\wx_base.vcxproj">
       <Project>{1c8436c9-dbaf-42be-83bc-cf3ec9175abe}</Project>
     </ProjectReference>
-    <ProjectReference Include="..\..\..\..\Externals\zlib\zlib.vcxproj">
-      <Project>{ff213b23-2c26-4214-9f88-85271e557e87}</Project>
-    </ProjectReference>
     <ProjectReference Include="..\..\..\Core\VideoCommon\VideoCommon.vcxproj">
       <Project>{3de9ee35-3e91-4f27-a014-2866ad8c3fe3}</Project>
     </ProjectReference>