Depth texture instead of pack/unpack.

[playcanvas]

Depth texture instead of pack/unpack.

[willeastcott]

For background, it's worth reading this:
http://blog.tojicode.com/2012/07/using-webgldepthtexture.html
I'm not convinced this buys us much but willing to be proved wrong.

[Maksims]

Believe with larger number of shadows in screen it will make a bit more difference, especially with wider PCF. We should try test in with few Point Light ShadowMaps with PCF.

[guycalledfrank]

Implemented it, but didn't found significant performance difference. Same on PC, very slightly faster on a Mali GPU tablet, slightly slower (why?) on iPhone 5s.
Not convinced we really need it now.