[get/set]RgbaUnsafe, slow before faster after

[guzba]

nim c --gc:arc -r -d:release -d:pixieNoSimd --debugger:native .\tests\benchmark_paints.nim

before:

GradientLinear vertical ............ 4.331 ms      4.348 ms    ±0.039  x1000
GradientLinear horizontal .......... 4.336 ms      4.349 ms    ±0.025  x1000
GradientLinear angular ............ 70.248 ms     70.907 ms    ±0.562    x71

after:

GradientLinear vertical ............ 0.592 ms      0.598 ms    ±0.010  x1000
GradientLinear horizontal .......... 1.218 ms      1.231 ms    ±0.022  x1000
GradientLinear angular ............ 67.623 ms     68.027 ms    ±0.393    x74

nim c --gc:arc -d:release -d:pixieNoSimd -r --debugger:native .\tests\benchmark_svg.nim

before:

svg decode ........................ 24.405 ms     24.664 ms    ±0.223   x203

after:

svg decode ........................ 19.164 ms     19.400 ms    ±0.240   x257

nim c --gc:arc -r -d:release -d:pixieNoSimd --debugger:native .\tests\benchmark_images_draw.nim

before:

draw small-on-big bmNormal ......... 0.024 ms      0.024 ms    ±0.000  x1000
draw small-on-big Smooth bmNormal .. 0.097 ms      0.100 ms    ±0.001  x1000
draw big-on-bigger bmNormal ........ 2.135 ms      2.148 ms    ±0.029  x1000
draw [scale 0.5] ................... 0.884 ms      0.886 ms    ±0.006  x1000
draw [scale 2] ..................... 8.682 ms      8.703 ms    ±0.029   x574
draw Smooth [x translate] .......... 8.207 ms      8.555 ms    ±0.102   x583
draw Smooth [y translate] .......... 9.192 ms      9.541 ms    ±0.101   x523
draw Smooth [x + y translate] ...... 8.911 ms      8.989 ms    ±0.059   x555
draw Smooth [rotate 45 deg] ........ 7.171 ms      7.330 ms    ±0.089   x682
draw mask Smooth [rotate 45 deg] ... 4.508 ms      4.552 ms    ±0.035  x1000
shadow (no offset) ................. 0.401 ms      0.406 ms    ±0.005  x1000
shadow (with offset) ............... 0.425 ms      0.429 ms    ±0.005  x1000

after:

draw small-on-big bmNormal ......... 0.010 ms      0.011 ms    ±0.001  x1000
draw small-on-big Smooth bmNormal .. 0.064 ms      0.065 ms    ±0.001  x1000
draw big-on-bigger bmNormal ........ 0.736 ms      0.739 ms    ±0.005  x1000
draw [scale 0.5] ................... 0.407 ms      0.411 ms    ±0.007  x1000
draw [scale 2] ..................... 3.284 ms      3.297 ms    ±0.022  x1000
draw Smooth [x translate] .......... 4.228 ms      4.240 ms    ±0.013  x1000
draw Smooth [y translate] .......... 4.539 ms      4.567 ms    ±0.035  x1000
draw Smooth [x + y translate] ...... 5.540 ms      5.559 ms    ±0.024   x897
draw Smooth [rotate 45 deg] ........ 5.753 ms      5.775 ms    ±0.031   x863
draw mask Smooth [rotate 45 deg] ... 2.719 ms      2.728 ms    ±0.014  x1000
shadow (no offset) ................. 0.316 ms      0.322 ms    ±0.007  x1000
shadow (with offset) ............... 0.339 ms      0.342 ms    ±0.005  x1000