proposal: text/v2

[hajimehoshi]

I've written a proposal for a brand new API for text rendering: text/v2 (#2454)!

https://docs.google.com/document/d/1sEuNezPULkR60jk89QHfQ7hhoyuPgKRfZ-CpqnoidWI/edit?usp=sharing

I aim to resolve these existing issues:

• text: Used fonts are never discarded #498
• text: Rendering texts correctly in Arabic/Thai/Hindi... #675
• Outline / Stroke text font #1937
• text: a utility function to render texts with a left-upper position #2143
• text: crash with an unhashable face #2669
• text/v2: utility APIs for text fields #2834
• OpenType's features and variants were not available

I apprecite your opinions. Thanks,