cache svgs and set svg bg to white (helps with github darkmode)

Signed-off-by: BarDweller <bardweller@gmail.com>

buildpack.md

@@ -402,35 +402,35 @@ Where:
 
 Given:
 
-<img src="http://tex.s2cms.ru/svg/%0AO%20%3D%0A%5Cbegin%7Bbmatrix%7D%0AA%2C%20%26%20B%20%5C%5C%0AC%2C%20%26%20D%0A%5Cend%7Bbmatrix%7D%0A" alt="
-O =
+![
+  O =
 \begin{bmatrix}
 A, &amp; B \\
 C, &amp; D
 \end{bmatrix}
-" />
+](img/matrix1.svg)
 
-<img src="http://tex.s2cms.ru/svg/%0AP%20%3D%0A%5Cbegin%7Bbmatrix%7D%0AE%2C%20%26%20F%20%5C%5C%0AG%2C%20%26%20H%0A%5Cend%7Bbmatrix%7D%0A" alt="
+![
 P =
 \begin{bmatrix}
 E, &amp; F \\
 G, &amp; H
 \end{bmatrix}
-" />
+"](img/matrix2.svg)
 
 It MUST follow that:
 
-<img src="http://tex.s2cms.ru/svg/%0A%5Cbegin%7Bbmatrix%7D%0AE%2C%20%26%20O%2C%20%26%20F%0A%5Cend%7Bbmatrix%7D%20%3D%20%0A%5Cbegin%7Bbmatrix%7D%0AE%2C%20%26%20A%2C%20%26%20B%2C%20%26%20F%20%5C%5C%0AE%2C%20%26%20C%2C%20%26%20D%2C%20%26%20F%20%5C%5C%0A%5Cend%7Bbmatrix%7D%0A" alt="
+![
 \begin{bmatrix}
 E, &amp; O, &amp; F
 \end{bmatrix} =
 \begin{bmatrix}
 E, &amp; A, &amp; B, &amp; F \\
 E, &amp; C, &amp; D, &amp; F \\
 \end{bmatrix}
-" />
+](img/matrix3.svg)
 
-<img src="http://tex.s2cms.ru/svg/%0A%5Cbegin%7Bbmatrix%7D%0AO%2C%20%26%20P%0A%5Cend%7Bbmatrix%7D%20%3D%20%0A%5Cbegin%7Bbmatrix%7D%0AA%2C%20%26%20B%2C%20%26%20E%2C%20%26%20F%20%5C%5C%0AA%2C%20%26%20B%2C%20%26%20G%2C%20%26%20H%20%5C%5C%0AC%2C%20%26%20D%2C%20%26%20E%2C%20%26%20F%20%5C%5C%0AC%2C%20%26%20D%2C%20%26%20G%2C%20%26%20H%20%5C%5C%0A%5Cend%7Bbmatrix%7D%0A" alt="
+![
 \begin{bmatrix}
 O, &amp; P
 \end{bmatrix} =
@@ -440,7 +440,7 @@ A, &amp; B, &amp; G, &amp; H \\
 C, &amp; D, &amp; E, &amp; F \\
 C, &amp; D, &amp; G, &amp; H \\
 \end{bmatrix}
-" />
+](img/matrix4.svg)
 
 Note that buildpack IDs are expanded depth-first in left-to-right order.
 

img/genmatrix.sh

@@ -0,0 +1,15 @@
+#!/bin/bash
+
+# A quick script to use tex.s2cms.ru to generate various matrix svgs.
+# (requests were previously inlined directly from buildpack.md, this allows the doc to be read if the remote side is down)
+
+wget -O matrix1.svg http://tex.s2cms.ru/svg/%0AO%20%3D%0A%5Cbegin%7Bbmatrix%7D%0AA%2C%20%26%20B%20%5C%5C%0AC%2C%20%26%20D%0A%5Cend%7Bbmatrix%7D%0A
+wget -O matrix2.svg http://tex.s2cms.ru/svg/%0AP%20%3D%0A%5Cbegin%7Bbmatrix%7D%0AE%2C%20%26%20F%20%5C%5C%0AG%2C%20%26%20H%0A%5Cend%7Bbmatrix%7D%0A
+wget -O matrix3.svg http://tex.s2cms.ru/svg/%0A%5Cbegin%7Bbmatrix%7D%0AE%2C%20%26%20O%2C%20%26%20F%0A%5Cend%7Bbmatrix%7D%20%3D%20%0A%5Cbegin%7Bbmatrix%7D%0AE%2C%20%26%20A%2C%20%26%20B%2C%20%26%20F%20%5C%5C%0AE%2C%20%26%20C%2C%20%26%20D%2C%20%26%20F%20%5C%5C%0A%5Cend%7Bbmatrix%7D%0A
+wget -O matrix4.svg http://tex.s2cms.ru/svg/%0A%5Cbegin%7Bbmatrix%7D%0AO%2C%20%26%20P%0A%5Cend%7Bbmatrix%7D%20%3D%20%0A%5Cbegin%7Bbmatrix%7D%0AA%2C%20%26%20B%2C%20%26%20E%2C%20%26%20F%20%5C%5C%0AA%2C%20%26%20B%2C%20%26%20G%2C%20%26%20H%20%5C%5C%0AC%2C%20%26%20D%2C%20%26%20E%2C%20%26%20F%20%5C%5C%0AC%2C%20%26%20D%2C%20%26%20G%2C%20%26%20H%20%5C%5C%0A%5Cend%7Bbmatrix%7D%0A
+
+# add background color.. (helps with github darkmode)
+sed -i -e's/<svg/<svg style="background-color:white"/' matrix1.svg
+sed -i -e's/<svg/<svg style="background-color:white"/' matrix2.svg
+sed -i -e's/<svg/<svg style="background-color:white"/' matrix3.svg
+sed -i -e's/<svg/<svg style="background-color:white"/' matrix4.svg