Take h_separation into account for label and icon positions (item_rect)

[joao-pedro-braz]

Description

This PR makes it so that when drawing an item_rect the h_separation is taken into account when computing the icon and label positions. The immediate effect is that TreeItem icons in the Editor are no longer drawn "hogging" the item's border, which was specially perceptible when using a Corner Radius greater than 3.

Comparisons

Default with Corner Radius 3

PR with Corner Radius 3

Default (RTL) with Corner Radius 3

PR (RTL) with Corner Radius 3

Default with Corner Radius 6

PR with Corner Radius 6

[joao-pedro-braz]

Also, is there any particular reason on why antialiasing is not enabled by default in the Editor Theme?

[YuriSizov]

h_separation is a constant for the distance between the icon and the label. It is not supposed to be applied to icon's initial position. So this is not taking it into account, this is adding it as an extra to fix the issue. I'm not sure this is a correct solution.

Instead, it's probably the stylebox that needs to be adjusted to expand further to the side.

Also, is there any particular reason on why antialiasing is not enabled by default in the Editor Theme?

Performance, it's not necessary but it's expensive, and Godot aims to be friendly to lower end hardware.

[joao-pedro-braz]

I guess the root problem then is that the TreeItem themselves don't have a StyleBox, right?
They only get one if they're focused and/or selected.

If we were to add a default StyleBox to them, then I guess we could solve this problem through the Content Margins.
What do you think?

[YuriSizov]

Yeah, that sounds like a good idea. Just need to make sure that in the default theme that stylebox doesn't do anything (so existing themes work as expected).

[joao-pedro-braz]

Yeah, that's a good idea.
Working on it right now!

[joao-pedro-braz]

Closing due to #74321, which implements this feature as discussed.