Refactor capture feature, fix data race reported by xcode

[halx99]

No description provided.

[halx99]

Use PBO:

capture screen, use PBO, cost: 10.270(ms)
capture screen, use PBO, cost: 9.750(ms)
capture screen, use PBO, cost: 9.045(ms)
capture screen, use PBO, cost: 9.232(ms)

No PBO:

capture screen, no PBO, cost: 17.235(ms)
capture screen, no PBO, cost: 18.457(ms)
capture screen, no PBO, cost: 17.306(ms)
capture screen, no PBO, cost: 16.876(ms)
capture screen, no PBO, cost: 17.132(ms)
capture screen, no PBO, cost: 17.157(ms)
capture screen, no PBO, cost: 18.863(ms)
capture screen, no PBO, cost: 18.560(ms)
capture screen, no PBO, cost: 17.864(ms)