Performance updates

[wouterlucas]

Stash of small performance updates:

• Removal of lots of redundant const creations
• Introduced a Texture Type to avoid more expensive instanceof checks
• Small refactor on flipY detection with RTT
• Reduced immediate GC pressure on node destroy by setting values to undefined over explicit delete
• Skip sanitizing custom data on node creation if inspector is disabled

Performance changes:

create: 60.24ms ±17.30 ->  50.07ms ±8.84
update: 30.66ms ±1.50 -> 28.87ms ±9.51
skipNth: 24.32ms ±7.00  -> 22.81ms ±5.22
select: 27.72ms ±12.20  -> 26.44ms ±15.72
swap: 28.14ms ±14.80 -> 24.16ms ±12.71
remove: 25.72ms ±5.20 -> 22.83ms ±5.25
createLots: 303.16ms ±141.40  -> 258.30ms ±50.86
append: 282.90ms ±36.70 -> 283.57ms ±191.68
clear: 14.76ms ±1.50 -> 14.59ms ±5.86