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/TextureCacheBase.cpp

@@ -242,7 +242,7 @@ void TextureCacheBase::ScaleTextureCacheEntryTo(TextureCacheBase::TCacheEntryBas
     return;
   }
 
-  u32 max = g_renderer->GetMaxTextureSize();
+  const u32 max = g_ActiveConfig.backend_info.MaxTextureSize;
   if (max < new_width || max < new_height)
   {
     ERROR_LOG(VIDEO, "Texture too big, width = %d, height = %d", new_width, new_height);