feat(packages): Add watermark package and trigger from build variable

Closes #154

packages/watermark.lua

@@ -0,0 +1,49 @@
+local base = require("packages.base")
+
+local package = pl.class(base)
+package._name = "watermark"
+
+local text = os.getenv("CASILE_WATERMARK") or "DRAFT"
+
+function package:_init(options)
+  if options.text then text = options.text end
+  base._init(self, options)
+  self:loadPackage("rotate")
+  self.class:registerHook("newpage", self.addWatermark)
+  self:addWatermark()
+end
+
+function package:addWatermark ()
+  local page = SILE.getFrame("page")
+  local frame = SILE.newFrame({
+    id = "watermark",
+    left = "left(page)+20",
+    right = "right(page)-20",
+    top = "top(page)+20",
+    bottom = "bottom(page)-20"
+  })
+  SILE.documentState.thisPageTemplate.frames.watermark = frame
+  SILE.typesetNaturally(frame, SILE.Commands["watermark:content"])
+end
+
+function package:registerCommands ()
+
+  SILE.registerCommand("watermark:content", function (_, _)
+    SILE.call("hbox")
+    SILE.call("skip", { height = "20%fh" })
+    SILE.call("center", {}, function ()
+      SILE.call("color", { color = "#fafafa" }, function ()
+        SILE.call("rotate", { angle = -35 }, function ()
+          SILE.call("font", { family = "TeX Gyre Heros", weight = 900, size = "8%fw" }, { text })
+        end)
+        SILE.call("skip", { height = "20%fh" })
+        SILE.call("rotate", { angle = -35 }, function ()
+          SILE.call("font", { family = "TeX Gyre Heros", weight = 900, size = "8%fw" }, { text })
+        end)
+      end)
+    end)
+  end)
+
+end
+
+return package

rules/rules.mk

@@ -430,6 +430,11 @@ $(FULLPDFS): %.pdf: $(BUILDDIR)/%.sil $$(call coverpreq,$$@) $$(call onpaperlibs
 		rm $*.tmp.pdf
 	fi
 
+ifneq ($(WATERMARK),)
+export CASILE_WATERMARK = $(WATERMARK)
+$(FULLPDFS): SILEFLAGS += --use packages.watermark
+endif
+
 DISTFILES += $(FULLPDFS)
 
 PANDOCTEMPLATE ?= $(CASILEDIR)/template.sil