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/VideoBackends/D3D/Render.cpp

@@ -1129,11 +1129,6 @@ void Renderer::SetInterlacingMode()
   // TODO
 }
 
-u32 Renderer::GetMaxTextureSize()
-{
-  return DX11::D3D::GetMaxTextureSize();
-}
-
 u16 Renderer::BBoxRead(int index)
 {
   // Here we get the min/max value of the truncated position of the upscaled framebuffer.