Update ImageMagick policy to allow the XC module

The presentation format recording scripts use the xc module (which
stands for "Constant Color" to generate placeholder images for slides
which are missing / failed to convert and a few other circumstances.

The XC module is safe to enable, since it doesn't involve reading any
files - it only generates single-color in memory images, and image sizes
(memory usage) are limited by other sections of the policy.

Author: kepstin

bbb-install.sh

@@ -498,6 +498,7 @@ main() {
   <policy domain="module" rights="none" pattern="*" />
   <policy domain="module" rights="read | write" pattern="{BMP,GIF,JPEG,PDF,PNG,TIFF,WEBP}"/>
   <policy domain="module" rights="read | write" pattern="{MPC}" stealth="true"/>
+  <policy domain="module" rights="read" pattern="{XC}"/>
   <policy domain="module" rights="write" pattern="{JSON,INFO,PNM,PS,SVG}"/>
   <!-- This policy sets the number of times to replace content of certain
        memory buffers and temporary files before they are freed or deleted. -->