Many things are coming to make the UI much much more responsive: * async pan and zoom * `will-change` support (avoid layers flattening) progressive tiles rend

Many things are coming to make the UI much much more responsive: * async pan and zoom * `will-change` support (avoid layers flattening) progressive tiles rendering * better texture allocation mechanism * CSS snapping * position: sticky * and many gaia code improvements (reflow and lags hunting)