Fix Target.new_to_descriptor for Lua 5.4

Author: rolandlo

src/vips/Target.lua

@@ -14,7 +14,9 @@ Target.vobject = function(self)
 end
 
 Target.new_to_descriptor = function(descriptor)
+    collectgarbage("stop")
     local target = vips_lib.vips_target_new_to_descriptor(descriptor)
+    collectgarbage("restart")
     if target == ffi.NULL then
         error("can't create output target from descriptor " .. descriptor)
     else