Support BGR subpixel layout on Windows

[dimhatz]

First of all, thank you for this nice library and the alacritty terminal that uses it.

This issue is for alacritty, but I think the appropriate place for it is here.

On windows, when using a BGR monitor / TV, the font rendering is very ugly (with noticeable color fringes).
On linux, this was fixed, according to alacritty/alacritty#4100 (AFAICT the fix was implemented in this library).

Adding some reference info, on how to detect which subpixel layout is used on Windows (in case it may be useful):

Microsoft docs

Investigation by BetterClearTypeTuner (2019)

Thank you again for your good work!

[avih]

Regardless if the subpixel orientation/order can or can't be detected automatically, it might be useful to expose a configuration option to allow the user to override/specify a value, with probably one of:

• Grayscale.
• RGB.
• BGR
• VRGB (vertical, if supported by Windows)
• VBGR (vertical, if supported by Windows)

And maybe also a "None" value, though I don't think many windows users prefer that except with fixed fonts - where this is not required anyway I think.