Support for referenced gradients

[SergeySlice]

Hi, The SVG file contains the follow

    <linearGradient id="linear-gradient" x1="49.93" y1="124.6" x2="49.93" y2="39.4" gradientTransform="matrix(1, 0, 0, -1, 0, 162)" gradientUnits="userSpaceOnUse">
        <stop offset="0" stop-color="#ff0"/>
        <stop offset="1" stop-color="#dd4814"/>
    </linearGradient>
    <linearGradient id="linear-gradient-2" x1="98.9" y1="133.54" x2="98.9" y2="85.29" xlink:href="#linear-gradient"/>
    <linearGradient id="linear-gradient-3" x1="99.1" y1="79.2" x2="99.1" y2="31.03" xlink:href="#linear-gradient"/>

And "linear-gradient" used as well as "linear-gradient-2" and "linear-gradient-3". Existing codes will not fully work. I propose follow patch (i don't know how to create pull request) createGradient.patch.zip

1. matrix multiplication data and ref
2. because of this ref should not be same as data.

Thanks for the good project! Regards

[SergeySlice]

To illustrate Ubuntu2.zip

[SergeySlice]

More correct it should be

nsvg__xformPremultiply(data->xform, ref->xform);

because of data->xform should be applied over referenced gradient.