[ci] use parallel builds

.drone.yml

@@ -1,29 +1,33 @@
 pipeline:
   inkscape:
+    group: prepare
     image: jess/inkscape
     commands:
       - apt-get update && apt-get -y install make
       - ./compile.sh inkscape
 
-  latex:
-    image: tianon/latex
-    commands:
-      - apt-get update && apt-get -y install wget
-      - ./compile.sh latex
-
   libreoffice:
+    group: prepare
     image: jess/libreoffice
     commands:
       - apk add --no-cache make
       - ./compile.sh libreoffice
 
   revealjs:
+    group: prepare
     image: node
     environment:
       - NODE_ENV=production
     commands:
       - ./compile.sh revealjs
 
+  latex:
+    group: build
+    image: tianon/latex
+    commands:
+      - apt-get update && apt-get -y install wget
+      - ./compile.sh latex
+
   publish:
     image: sbruder/drone-netlify
     secrets: [ "netlify_key" ]