Default opacity channel should be A (playcanvas#1117)

* default opacity channel = 'a'

Fix iOS after recent shader chunk changes

Update README.md

- Use particleDistance values in vbToSort array instead of indices, a…

…rray lookups in sort function are expensive (playcanvas#1072)

- Use TypedArray.set instead of manually copying elements into vbOld - TypedArray.set does an optimised copy
- Unroll for loop in sorting code, saving an inner loop and a few multiplies per vertex

Fix lint warning

Merge pull request playcanvas#1055 from playcanvas/feature/fonts-mult…

…iple-textures

Support multiple textures per font asset

[FIX] Fix resuming sounds from the same spot after pausing

Fix model and element component docs

Fix BatchManager doc

Added pc.Application#batcher docs