PDF written highlights: fix boxes, trash cached tiles

TileCacheItem: add created_ts property.
Document: manage a tile_cache_validity_ts and ignore
older cached tiles.
This timestamps is updated when highlights are written
as annotations in, or deleted from, the PDF, so we can
get the most current rendered bitmap from MuPDF and
avoid highlight ghosts on old tiles.
Save this timestamp in doc settings so older cached to
disk tiles will also be ignored across re-openings.
Bump base for: mupdf.lua: update frontend pboxes with
MuPDF adjusted ones.

frontend/document/tilecacheitem.lua

@@ -17,6 +17,7 @@ function TileCacheItem:totable()
         size = self.size,
         pageno = self.pageno,
         excerpt = self.excerpt,
+        created_ts = self.created_ts,
         persistent = self.persistent,
         bb = {
             w = self.bb.w,
@@ -51,6 +52,7 @@ function TileCacheItem:fromtable(t)
     self.size = t.size
     self.pageno = t.pageno
     self.excerpt = t.excerpt
+    self.created_ts = t.created_ts
     self.persistent = t.persistent
     self.bb = Blitbuffer.fromstring(t.bb.w, t.bb.h, t.bb.fmt, t.bb.data, t.bb.stride)
 end