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/D3D12/D3DBase.h

@@ -121,8 +121,6 @@ const std::string PixelShaderVersionString();
 const std::string GeometryShaderVersionString();
 const std::string VertexShaderVersionString();
 
-unsigned int GetMaxTextureSize();
-
 HRESULT SetFullscreenState(bool enable_fullscreen);
 bool GetFullscreenState();