Support for RTL text

[tocsoft]

Add option to text layout engine to support rendering 'right to left' instead of the current 'left to right'.

This will require new option on RendererOptions to allow configuring 'right to left`
and updating

Fonts/src/SixLabors.Fonts/TextLayout.cs

Line 25 in 4ee628b

public ImmutableArray<GlyphLayout> GenerateLayout(string text, RendererOptions options)

to layout the characters in the corrent 'right to left' style.

[JimBobSquarePants]

While I've added a Bidi implementation (not wired up or publicly exposed) in #160 I'm really not happy with the shape of the API it brings.

I want to investigate this alternative.
https://github.com/Tehreer/SheenBidi

LayoutFarm Typography is currently using a C# version that appears to be based on https://archive.codeplex.com/?p=sheenbidi written by the same author as the C version.