Saving R8_UNORM as a BMP fails

[walbourn]

If you attempt to write a BMP file using the WIC code starting with a DXGI_FORMAT_R8_UNORM image, it fails with an 'invalid argument' error.

[walbourn]

When you attempt to write the BMP, the WIC BMP native codec can't directly support GUID_WICPixelFormat8bppGray (the WIC format that maps from DXGI_FORMAT_R8_UNORM). Instead, it returns that it can support GUID_WICPixelFormat8bppIndexed. This is expected and supported (to need a format conversion to write), but it ultimately fails on this line of code:

hr = FC->Initialize(frame, convertGUID, _GetWICDither(flags), nullptr, 0,
    WICBitmapPaletteTypeCustom);

The problem is that WICBitmapPaletteTypeCustom means "use the palette object passed in the fourth parameter" which is always nullptr. The fix is to change it to:

hr = FC->Initialize(frame, convertGUID, _GetWICDither(flags), nullptr, 0,
    WICBitmapPaletteTypeMedianCut);

This pattern is used in many places in DirectXTex but it turns out that there's almost no case where it actually tries to convert to a paletted format--DXGI doesn't support them. It only comes up in the case where a WIC codec writer maps the input format to an indexed format. It also turned out that this isn't hit by my test code either.

[walbourn]

Cleaned up all of DirectXTex (and DirectXTK/DirectXTK12) to use WICBitmapPaletteTypeMedianCut instead of WICBitmapPaletteTypeCustom. Fixed in commit 0a85ac75a04dc02d29d7a658b6f985efeb81dd3c.

Also updated the test suite to do an exhaustive 'transcode' test to exercise more target pixel formats for the WIC writer.

[walbourn]

Fix included in the July 2017 release