optimize: ignore z for sprite transformations
don't count root node
SpriteTools doc
minor
allow to define [min, max] frame interval
assert set alpha with no texture/color
update / clean up renderers
pass AnimSequence to callback
replace propageStateFromRoot with iterative version
further optimize DisplayListRenderer
mark Spatial as freed after calling free()
warp rotation to [0,360]
work on sprite alpha property
refactor alpha state / blending modes