JavaFX metal rendering pipeline for macOS

[Kevin Rushforth]

We just pushed an early prototype of a new JavaFX Metal rendering 
pipeline for macOS to a new "metal" branch of jfx-sandbox. We called it 
"metal" with no bug ID or username prefix, to highlight that it is more 
than an experiment: It is intended to be the branch where we will 
develop the metal port. Future experimental work can be done in other 
branches and those would have a username prefix.

We're not necessarily looking for contributions at this point, but if 
anyone has early feedback about the approach, or wants to "kick the 
tires" by building it themselves, that would be fine.

Several things are hard-coded at the moment (such as a hack to force 
uploading painter since the needed glass changes haven't been done, and 
a hard coded background color), and by no means does everything work yet 
(e.g., no text, no decora shaders, no 3D), but basic primitive work.

-- Kevin