Add support for Color Emoji support

[dotMorten]

Currently when you render emojis in WPF, they are rendered black'n'white, and not rendering with the correct emoji color.
It would be good if we could update the text rasterizer renderers to render with the correct color.
There's a proof of concept implementation here: https://github.com/samhocevar/emoji.wpf, so perhaps @samhocevar would be willing to contribute? (I'm guessing this would be done on the DirectWrite level though)

Screenshot from Emoji.WPF

Documentation how this works from https://blogs.windows.com/buildingapps/2017/06/06/using-color-fonts-beautiful-text-icons/#Crwe2VHsjP5PgAXS.97 (thanks @mikedn):

Also for discussion: Should we have a IsColorFontEnabled property that turns this behavior on/off, and should it be default like in UWP (behavior change). Also only 8.1+ uses the D2D1_DRAW_TEXT_OPTIONS_ENABLE_COLOR_FONT flag, so I'm assuming the behavior would always be black/white on Win7 and 8.0

[mikedn]

I'm guessing this would be done on the DirectWrite level though

Yes, DWrite should already handle this if you tell it so:

https://blogs.windows.com/buildingapps/2017/06/06/using-color-fonts-beautiful-text-icons

[dotMorten]

@mikedn Thanks. Info added to issue

[mikedn]

Ahm, but then WPF doesn't actually use D2D, it only uses DWrite. That might mean that to implement this you need to do some more involved work in WPF's rendering code. I think, it's been a while since I looked at WPF's text rendering...

[fabiant3-zz]

@dotMorten - This looks like a great feature addition. Unfortunately, at this time we are unsure of the exact time when the native layers will be open sourced. I will move this to future for now.

[be5invis]

@mikedn DWRITE already has complete color font support. Just use IDWriteFactory4::TranslateColorGlyphRun

[mikedn]

@be5invis I know, but as I mentioned above DWrite isn't the problem. The problem is WPF's rendering which is unlikely to support colored glyph run. It's been a while since I looked at that code but I don't expect it to be easy to add such support.

[be5invis]

@mikedn

It is not that complex. Color glyph conversion lies at the very final end so you can just wrap your original draw call into something like this: https://github.com/Microsoft/Windows-universal-samples/blob/master/Samples/DWriteColorGlyph/cpp/CustomTextRenderer.cpp (this one supports all latest DWRITE's color formats, even SVG).

[be5invis]

Other notable features of Emoji support:

• Emoji needs glyphing (for ZWJ and skin tones)
• Handle VS15 and VS16: when meeting a VS16, prioritize Emoji fonts on the font fallback sequence; When meeting VS15, remove Emoji font from the fallback sequence
• Keycap combinations

[be5invis]

cc. @fdwr