simpler source position when not rotated or scaled

[guzba]

before:

pixie draw normal .................. 0.626 ms      0.648 ms    ±0.023  x1000
pixie draw overwrite ............... 0.627 ms      0.648 ms    ±0.026  x1000
pixie draw basic ................... 0.461 ms      0.477 ms    ±0.024  x1000
pixie draw mask .................... 0.853 ms      0.895 ms    ±0.050  x1000

after:

pixie draw normal .................. 0.586 ms      0.643 ms    ±0.058  x1000
pixie draw overwrite ............... 0.586 ms      0.608 ms    ±0.023  x1000
pixie draw basic ................... 0.477 ms      0.492 ms    ±0.016  x1000
pixie draw mask .................... 0.757 ms      0.793 ms    ±0.039  x1000

---

before:

Total time: 3256.025ms
Total diff: 0.249%

after:

Total time: 3096.062ms
Total diff: 0.249%