VideoBackends: Move max texture size to VideoConfig

This stops the virtual method call from within the Renderer constructor.

The initialization here for GL had to be moved to VideoBackend, as the
Renderer constructor will not have been executed before the value is
required.

Source/Core/VideoCommon/VideoConfig.h

@@ -173,6 +173,8 @@ struct VideoConfig final
     // TODO: merge AdapterName and Adapters array
     std::string AdapterName;  // for OpenGL
 
+    u32 MaxTextureSize;
+
     bool bSupportsExclusiveFullscreen;
     bool bSupportsDualSourceBlend;
     bool bSupportsPrimitiveRestart;