Add LCD subpixel rendering

[adamjs]

LCD subpixel rendering (eg, ClearType et al) can be used to improve the perceived contrast of glyph shapes on lower-DPI screens, unfortunately it suffers from a host of drawbacks:

1. Expensive and slow since we need to render back-to-front and sample the background when compositing.
2. Doesn't work well when animating text in a composite layer (texels are not in a 1:1 mapping with monitor LCD sub-pixels).
3. Doesn't work well when user is using a non-native monitor resolutions (common in games).
4. Not needed with high-DPI monitors (Apple has dropped support for LCD subpixel rendering macOS Mojave and beyond).

Regardless it is a good feature to have, and we can allow users to selectively enable it behind a Config setting.

[sirgru]

Related: #194

[wolfield]

Since the majority of the desktop displays are still under 120 dpi lack of the subpixel rendering is a complete showstopper for desktop applications of Ultralight.

Washed away fonts and user experience with usability do not mix. You just can't make feature dense interfaces since you are forced to make fonts larger to make them decently readable.

So for now I move on to Electron with pixel perfect fonts. Ultralight is so much nicer for interop with C++, but I'd rather provide proper experience to the user and write a ton of boilerplate around native modules, unfortunately, than use Ultralight.

[SupinePandora43]

I've heard this can be implemented using Dual Source Blending, is that true?