add support for shadow colors in svg

[asturur]

closes #2098

canvas

exported svg

resulting svg

<?xml version="1.0" encoding="UTF-8" standalone="no" ?><!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" version="1.1" width="700" height="600" xml:space="preserve"><desc>Created with Fabric.js 1.5.0</desc><defs></defs><filter id="SVGID_1" y="-80%" height="260%" x="-80%" width="260%" >
    <feGaussianBlur in="SourceAlpha" stdDeviation="10" ></feGaussianBlur>
    <feOffset dx="10" dy="10"  result="offsetBlur" ></feOffset>
    <feFlood flood-color="blue"/>
    <feComposite in2="offsetBlur" operator="in" />
    <feMerge>
        <feMergeNode></feMergeNode>
        <feMergeNode in="SourceGraphic"></feMergeNode>
    </feMerge>
</filter>
<rect x="-25" y="-25" rx="0" ry="0" width="50" height="50" style="stroke: none; stroke-width: 1; stroke-dasharray: none; stroke-linecap: butt; stroke-linejoin: miter; stroke-miterlimit: 10; fill: red; fill-rule: nonzero; opacity: 1;filter: url(#SVGID_1);" transform="translate(179.75 179.75) scale(5.09 5.09)"/>
<filter id="SVGID_2" y="-45%" height="190%" x="-45%" width="190%" >
    <feGaussianBlur in="SourceAlpha" stdDeviation="5" ></feGaussianBlur>
    <feOffset dx="10" dy="10"  result="offsetBlur" ></feOffset>
    <feFlood flood-color="rgba(0,0,0,0.3)"/>
    <feComposite in2="offsetBlur" operator="in" />
    <feMerge>
        <feMergeNode></feMergeNode>
        <feMergeNode in="SourceGraphic"></feMergeNode>
    </feMerge>
</filter>
<circle cx="0" cy="0" r="40" style="stroke: none; stroke-width: 1; stroke-dasharray: none; stroke-linecap: butt; stroke-linejoin: miter; stroke-miterlimit: 10; fill: rgb(0,255,0); fill-rule: nonzero; opacity: 0.5;filter: url(#SVGID_2);" transform="translate(437.5 91.5) "/>
</svg>

[kangax]

👍