Reworked Screenshot saving.

Now OGL doesn't rely on WX for PNG saving.
FlipImageData supports (pixel data len > 3) now.
TextureToPng is now in ImageWrite.cpp/h
Video Common depends on zlib and png.
D3D no longer depends on zlib and png.

Source/Core/VideoCommon/Src/ImageWrite.h

@@ -9,6 +9,7 @@
 
 bool SaveTGA(const char* filename, int width, int height, void* pdata);
 bool SaveData(const char* filename, const char* pdata);
+bool TextureToPng(u8* data, int row_stride, const char* filename, int width, int height, bool saveAlpha = true);
 
 #endif  // _IMAGEWRITE_H