support stroke dash offset function about thorvg

[mgrudzinska]

<svg viewBox="-10 0 50 20" xmlns="http://www.w3.org/2000/svg">
  <!-- No dash array -->
  <line x1="0" y1="1" x2="30" y2="1" stroke="black" />

  <!-- No dash offset -->
  <line x1="0" y1="3" x2="30" y2="3" stroke="black" stroke-dasharray="3.4 1.3 2 0.5" stroke-linecap="butt" />


  <line x1="0" y1="5" x2="30" y2="5" stroke="blue"
        stroke-dasharray="3.4 1.3 2 0.5" stroke-dashoffset="1.2" />

  <line x1="0" y1="6.1" x2="30" y2="6.1" stroke="blue"
        stroke-dasharray="3.4 1.3 2 0.5" stroke-dashoffset="8.4" />


  <line x1="0" y1="8" x2="30" y2="8" stroke="blue"
        stroke-dasharray="3.4 1.3 2 0.5" stroke-dashoffset="4.1" />

  <line x1="0" y1="9.1" x2="30" y2="9.1" stroke="blue"
        stroke-dasharray="3.4 1.3 2 0.5" stroke-dashoffset="11.3" />


  <line x1="0" y1="11" x2="30" y2="11" stroke="blue"
        stroke-dasharray="3.4 1.3 2 0.5" stroke-dashoffset="5.7" />

  <line x1="0" y1="12.1" x2="30" y2="12.1" stroke="blue"
        stroke-dasharray="3.4 1.3 2 0.5" stroke-dashoffset="12.9" />
</svg>

result from tvg after my changes (as expected):

need to test negative values of offset and will be ready :)

[hermet]

Great, I'm looking forward to the result. :-)

[mgrudzinska]

@hermet
changing api like this Result stroke(const float* dashPattern, uint32_t cnt, float offset = 0.0f) noexcept;
will cause a problem, because we have already Result stroke(uint8_t r, uint8_t g, uint8_t b, uint8_t a = 255) noexcept;

so calling stroke(0, 255, 0) is ambiguous.
I'm not sure how to solve this... we could remove the def value of a but I assume that it's a common api, so probably not the best idea. so a new Result dashOffset(float offset) api?

[hermet]

how about -> strokeDash(dashPattern, patternCnt, offset) ?

[hermet]

ok, you already pushed a patch. Reverting alpha value might be better to go for now.