prepare for more elaborate caching policy

we allow for bigger render buffers, so the cache now returns
the ID (hash) of the relevant cashed tile PLUS offsets into that
tile, pointing to the coordinates where the requested frame
has its origin.

unireader.lua

@@ -189,7 +189,7 @@ function UniReader:draworcache(no, zoom, offset_x, offset_y, width, height, gamm
 		-- so give it more ttl.
 		self.cache[hash].ttl = self.cache_max_ttl
 	end
-	return hash
+	return hash, 0, 0
 end
 
 -- calculate a hash for our current state
@@ -358,12 +358,14 @@ end
 -- render and blit a page
 function UniReader:show(no)
 	local slot
+	local offset_x -- resulting display offset
+	local offset_y
 	if self.globalzoommode ~= self.ZOOM_BY_VALUE then
-		slot = self:draworcache(no,self.globalzoommode,self.offset_x,self.offset_y,width,height,self.globalgamma,self.globalrotate)
+		slot, offset_x, offset_y = self:draworcache(no,self.globalzoommode,self.offset_x,self.offset_y,width,height,self.globalgamma,self.globalrotate)
 	else
-		slot = self:draworcache(no,self.globalzoom,self.offset_x,self.offset_y,width,height,self.globalgamma,self.globalrotate)
+		slot, offset_x, offset_y = self:draworcache(no,self.globalzoom,self.offset_x,self.offset_y,width,height,self.globalgamma,self.globalrotate)
 	end
-	fb.bb:blitFullFrom(self.cache[slot].bb)
+	fb.bb:blitFrom(self.cache[slot].bb, 0, 0, offset_x, offset_y, width, height)
 	if self.rcount == self.rcountmax then
 		print("full refresh")
 		self.rcount = 1