Fix exporting R8 files as PNG using libpng · microsoft/DirectXTex@e4aad30

enumerator 'D3D12_RENDER_PASS_BEGINNING_ACCESS_TYPE_DISCARD' in switch of enum 'D3D12_RENDER_PASS_BEGINNING_ACCESS_TYPE' is not handled

enumerator 'D3D12_RENDER_PASS_BEGINNING_ACCESS_TYPE_PRESERVE' in switch of enum 'D3D12_RENDER_PASS_BEGINNING_ACCESS_TYPE' is not handled

enumerator 'D3D12_RENDER_PASS_BEGINNING_ACCESS_TYPE_NO_ACCESS' in switch of enum 'D3D12_RENDER_PASS_BEGINNING_ACCESS_TYPE' is not handled

enumerator 'D3D12_RENDER_PASS_ENDING_ACCESS_TYPE_DISCARD' in switch of enum 'D3D12_RENDER_PASS_ENDING_ACCESS_TYPE' is not handled

enumerator 'D3D12_RENDER_PASS_ENDING_ACCESS_TYPE_PRESERVE' in switch of enum 'D3D12_RENDER_PASS_ENDING_ACCESS_TYPE' is not handled

enumerator 'D3D12_RENDER_PASS_ENDING_ACCESS_TYPE_NO_ACCESS' in switch of enum 'D3D12_RENDER_PASS_ENDING_ACCESS_TYPE' is not handled

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