Fix RichTextLabel scrollbar when too small to show it

[kitbdev]

• Related Fix Control minimum size check too aggressive #116343

When the RTL is too small to show the scrollbar, it isn't handled well, and the scollbar may be visible or not. When changing the size it flickers.

Before:

rtl-scrollbar-small-handling.mp4

After:

rtl-scrollbar-small-handling-fix.mp4

It also had issues when in a BoxContainer and it was too small to show, such as the script editor's error text at minimum width. It was escaping the BoxContainer rect.

Before (depending on the speed you do it, this is if you minimize it slowly):

After:

Now the scrollbar just doesn't show if too small.
This check prevents p_width - scroll_w from being 0 or less which caused issues.

[Repiteo]

Thanks!