RichTextLabel: BBCode [color] tags break DynamicFont extra Char spacing

[cwook]

Godot version:
3.2 beta2 official, tested with both GLES2 and GLES3 renderer.
OS/device including version:
Windows 7 Ultimate, 64-bit
Issue description:
In RichTextLabel bbcode, a [color] tag will replace the char spacing specified in the given DynamicFont with the default char spacing.

Steps to reproduce:

1. Add a RichTextLabel and enable bbcode.
2. Add some bbcode using color tags:

[center]|||||[/center]
[color=#ff00ff00]||||||[/color][color=#ffffff00]||||||[/color][color=#ffff0000]||||||[/color]
||[color=#ffffffff]|||||||[/color]

3. Add a custom font and adjust the extra spacing for char.

Minimal reproduction project:
DemoRTLDynamicFontCharSpacing.zip

[Calinou]

Related to #31089.

[akien-mga]

Fixed by #68576.