Add svg source for the black hole's texture

assets/accretion_disk.svg

@@ -0,0 +1,80 @@
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+
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+   xmlns:cc="http://creativecommons.org/ns#"
+   xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
+   xmlns:svg="http://www.w3.org/2000/svg"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:sodipodi="http://sodipodi.sourceforge.net/DTD/sodipodi-0.dtd"
+   xmlns:inkscape="http://www.inkscape.org/namespaces/inkscape"
+   width="793.70081"
+   height="1122.5197"
+   viewBox="0 0 210 297"
+   version="1.1"
+   id="svg8"
+   sodipodi:docname="accretion_disk.svg"
+   inkscape:version="0.92.5 (2060ec1f9f, 2020-04-08)">
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+       inkscape:collect="always"
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+       id="filter1006"
+       x="-0.047823932"
+       width="1.0956479"
+       y="-0.048177369"
+       height="1.0963547">
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+     id="base"
+     pagecolor="#ffffff"
+     bordercolor="#666666"
+     borderopacity="1.0"
+     inkscape:pageopacity="0"
+     inkscape:pageshadow="2"
+     inkscape:zoom="0.65297002"
+     inkscape:cx="638.40178"
+     inkscape:cy="466.56457"
+     inkscape:document-units="px"
+     inkscape:current-layer="layer1"
+     showgrid="false"
+     objecttolerance="1"
+     inkscape:window-width="2048"
+     inkscape:window-height="1076"
+     inkscape:window-x="-4"
+     inkscape:window-y="4"
+     inkscape:window-maximized="1"
+     inkscape:pagecheckerboard="true"
+     units="px" />
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+     id="metadata5">
+    <rdf:RDF>
+      <cc:Work
+         rdf:about="">
+        <dc:format>image/svg+xml</dc:format>
+        <dc:type
+           rdf:resource="http://purl.org/dc/dcmitype/StillImage" />
+        <dc:title></dc:title>
+      </cc:Work>
+    </rdf:RDF>
+  </metadata>
+  <g
+     inkscape:label="Layer 1"
+     inkscape:groupmode="layer"
+     id="layer1">
+    <path
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+       id="path828"
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+</svg>