use translate2d instead of top/left for tiles when hw-accelerated

this fixes gaps between tiles in Safari and makes iOS Safari butter
smooth when zooming
make sure tiles in iOS Safari are not hw-accelerated layers
horrible hacks to fix iOS Safari bugging out hw-acceleration
move the hack to tile container reset
position the hack absolutely
fix build
size-based tile layer HW hack; use top/left for tiles everywhere
fix blurry non-retina tiles on retina iOS
move Safari hacks to CSS, do not optimize contrast in Chrome
remove leftover code
Merge pull request #2377 from Leaflet/accel

Smarter hardware acceleration