Restore BitmapText anchor point

Since Phaser 2, BitmapText lost its anchor point.

Checked code in previous versions and just changed it a bit to fix the new version.

I was not sure if pivot correction should be made on preUpdate or postUpdate, felt like it was better in preUpdate but maybe it should be changed.

First time using GIT to contribute to a project, so I'm sorry if I made any mistake in the process.