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/Render.h

@@ -61,8 +61,6 @@ public:
 
   bool CheckForResize();
 
-  u32 GetMaxTextureSize() override;
-
   static D3D12_BLEND_DESC GetResetBlendDesc();
   static D3D12_DEPTH_STENCIL_DESC GetResetDepthStencilDesc();
   static D3D12_RASTERIZER_DESC GetResetRasterizerDesc();