Fixes for rendering splash image

[jwmcglynn]

Implement features required for Donner to render image.

• [x] <feGaussianBlur>, MVP of #151
• [x] Gradient positioning (see yellow bursts around lighting/text hit)

Here is how donner renders it: src/svg/renderer/testdata/golden/donner_splash.png

[jwmcglynn]

Snapshot of donner's rendering, notice the unblurred lightning glows and mispositioned lightning strike glows.

[jwmcglynn]

These are the gradients, maybe an issue with gradientUnits="userSpaceOnUse"

  <radialGradient id="radial-gradient-logo-hit-burst-1" cx="601.99" cy="300.5" fx="601.99" fy="300.5" r="25.74" gradientTransform="translate(20.87 99.61) scale(1 .54)" gradientUnits="userSpaceOnUse">
    <stop offset="0" stop-color="#f7971f"/>
    <stop offset="1" stop-color="#f7971f" stop-opacity="0"/>
  </radialGradient>
  <radialGradient id="radial-gradient-logo-hit-burst-2" cx="591.99" cy="304.55" fx="591.99" fy="304.55" r="25.74" gradientTransform="translate(99.58 112.71) scale(.79 .54)" xlink:href="#radial-gradient-logo-hit-burst-1"/>
  <radialGradient id="radial-gradient-logo-hit-burst-3" cx="298.93" cy="104.9" fx="298.93" fy="104.9" r="25.74" gradientTransform="translate(339.12 54.93) rotate(90) scale(1 .54)" xlink:href="#radial-gradient-logo-hit-burst-1"/>

[jwmcglynn]

Fixed in 9f145f9bca878eebe2f02e58c21ea0a763070393