NanoVG and OUI ports

[ketmar via Digitalmars-d-announce]

also, i'm slowly working on Yet Another Immediage GUI Library 
based on Blendish and simplified flex box model.

for now, it can do layouting (with line wrapping, and determining 
minimal window size) and has some basic controls, like buttons, 
labels, checkboxes.

also, i changed NanoVG API a little. mostly got rid of `nvg` 
prefix for functions accepting `NVGcontext*` ('cmon, we have 
modules and UFCS in D!), and changed `NVGcontext*` arg to simply 
`NVGContext` (note capital "C").

btw, it is GPLed now. bad news for GPL haters.