Fix consistency of GradientTexture changes

[KoBeWi]

Gradient texture queues an update when a property changes, EXCEPT in set_gradient() in GradientTexture2D, which calls the update immediately and emits the changed signal twice. Another weird thing is that gradient texture changes would queue the update and the changed signal would be emitted in the _update() method. That's not how this signal works.

This PR removes the inconsistency and _queue_update() is now called for every change, but the deferred emit_changed() is removed. Instead the signal is emitted right in the setter. I also added a public (not exposed) method update_now(), which forces the pending texture update to be done right now. It's automatically called when you attempt to get the image.

[YuriSizov]

EXCEPT in set_gradient() in GradientTexture2D

Just for future readers, this except comes from #60361.

[KoBeWi]

The interesting part is that this change wasn't done in GradientTexture1D.
EDIT:
Right, it was already like that...

[akien-mga]

Thanks!

[Calinou]

Does this PR supersede #80855?

[KoBeWi]

Yes.
EDIT:
Not entirely. I'll leave a comment.

[kleonc]

This PR removes the inconsistency and _queue_update() is now called for every change

Some leftovers (for whatever reason it's connecting to GradientTexture1D::_update instead of GradientTexture1D::_queue_update):

godot/scene/resources/gradient_texture.cpp

Lines 66 to 72 in bc88dca

	if (gradient.is_valid()) {
	gradient->disconnect_changed(callable_mp(this, &GradientTexture1D::_update));
	}
	gradient = p_gradient;
	if (gradient.is_valid()) {
	gradient->connect_changed(callable_mp(this, &GradientTexture1D::_update));
	}