Update gallery images: reduce resolution, add Mandelbrot

[MatthewPeterKelly]

Follow-up to #130, which added the gallery and described how it works.

Reduce the resolution so that the README loads faster.

Add an entry for the Mandelbrot set.

[MatthewPeterKelly]

Add the Mandelbrot set... but at a zoomed in a bit.

{
  "Mandelbrot": {
    "image_specification": {
      "resolution": [1000, 500],
      "center": [-0.006893157175705378, 0.7381186687313246],
      "width": 0.05
    },
    "convergence_params": {
      "escape_radius_squared": 5.0,
      "max_iter_count": 12000,
      "refinement_count": 5
    },
    "color_map": {
      "keyframes": [
        {
          "query": 0.0,
          "rgb_raw": [5, 0, 30]
        },
        {
          "query": 0.87,
          "rgb_raw": [20, 0, 90]
        },
        {
          "query": 0.94,
          "rgb_raw": [20, 60, 120]
        },
        {
          "query": 0.97,
          "rgb_raw": [0, 50, 240]
        },
        {
          "query": 1.0,
          "rgb_raw": [230, 245, 255]
        }
      ],
      "lookup_table_count": 1024,
      "background_color_rgb": [0, 0, 0],
      "histogram_bin_count": 512,
      "histogram_sample_count": 100000
    },
    "render_options": {
      "downsample_stride": 1,
      "subpixel_antialiasing": 2
    }
  }
}