Fix books not dimming the world behind them

[digi2303]

Opening a book on 1.21 doesn't dim the world behind it the way it does on 1.20.

GuiBasicBook has an empty renderMenuBackground override. It was added in c9fe2ea and hasn't shipped in a release, so it only affects this branch.

Deleting it isn't quite enough though. On 1.21 Screen.render calls renderBackground itself, which it didn't do on 1.20, so the background got drawn a second time over the finished book. And the old 1.20 gradient is still there as renderTransparentBackground, nothing just calls it anymore.

So this makes renderBackground a no-op and draws the background once in render(), before the book. The flush() is needed because fillGradient is batched while BookBlit draws immediately, same as drawEntityOnScreen already does.

renderBlurredBackground is unreachable now. The blur stays off anyway, happy to remove it if you'd rather.