Bangle.js 2 graphics speed improvements

Just a note to say in the 'Cutting edge' builds (2v15.47 and later) I've made some pretty decent improvements to graphics.

• A clear screen command has gone down from 24ms to 2ms
  
• The Bangle can now send data to the LCD in the background,so it can be executing JS while the screen is updating
  
  

Hopefully this won't introduce any issues, but the effect on scrolling especially in the launcher is pretty big!