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.h

@@ -62,8 +62,6 @@ public:
 
   bool CheckForResize();
 
-  u32 GetMaxTextureSize() override;
-
 private:
   void BlitScreen(TargetRectangle src, TargetRectangle dst, D3DTexture2D* src_texture,
                   u32 src_width, u32 src_height, float Gamma);