Document GradientTexture updates being deferred

[Calinou]

I don't know if a new force_update() method could be added, but that's proposal territory.

• This closes get_image() gives wrong result on GradientTexture2D  #66627.

[KoBeWi]

#81137 ensured that get_image() gives correct results. However the update is still deferred, so unless you force it (which get_image() does), the actual update will happen at the end of the frame.

I'm not sure about the "after 2 frames" part. Deferred class still happen in the same frame, just after everything else (see #77688), so the texture might be correct immediately. If that is really the case, this PR can be closed.

EDIT:
By "correct" I mean display correctly.

[YuriSizov]

Deferred class still happen in the same frame, just after everything else

This still means that you need to wait until the next frame to access the image (or use a deferred call in a script as well), does it not?

[KoBeWi]

As I said, get_image() doesn't require waiting. The update itself is deferred, but not sure if it affects anything. Maybe the texture displays old data for a single frame.

[YuriSizov]

Ah, I see what's going on. get_image() specifically forces an update. Then indeed, this shouldn't require you wait for the next frame. It should be available immediately, unless RenderingServer::texture_replace is somehow deferred. But it doesn't appear this way.

I'm going to close this PR then. Thanks!