Add Auxiliary files for JPEG/PNG support on Linux (#407)

Auxiliary/DirectXTexPNG.h

@@ -0,0 +1,33 @@
+//--------------------------------------------------------------------------------------
+// File: DirectXTexPNG.h
+//
+// DirectXTex Auxilary functions for using the PNG(http://www.libpng.org/pub/png/libpng.html) library
+//
+// For the Windows platform, the strong recommendation is to make use of the WIC
+// functions rather than using the open source library. This module exists to support
+// Windows Subsystem on Linux.
+//
+// Copyright (c) Microsoft Corporation.
+// Licensed under the MIT License.
+//--------------------------------------------------------------------------------------
+
+#pragma once
+
+#include "DirectXTex.h"
+
+
+namespace DirectX
+{
+    HRESULT __cdecl GetMetadataFromPNGFile(
+        _In_z_ const wchar_t* szFile,
+        _Out_ TexMetadata& metadata);
+
+    HRESULT __cdecl LoadFromPNGFile(
+        _In_z_ const wchar_t* szFile,
+        _Out_opt_ TexMetadata* metadata,
+        _Out_ ScratchImage& image);
+
+    HRESULT __cdecl SaveToPNGFile(
+        _In_ const Image& image,
+        _In_z_ const wchar_t* szFile);
+}