Kerning pairs not available in DynamicFont

[Chaosinism]

Godot version:
Godot v3.1.1 Stable
Godot v3.2 Stable

OS/device including version:
Windows 10

Issue description:

Different from BitmapFont, the kerning is rendered identically for every character pair in DynamicFont.
It seems that the next character in the text (p_next) is not really taken into consideration in functions such as DynamicFont::get_char_size and DynamicFontAtSize::draw_char (unlike BitmapFont::get_char_size which queries the kerning pair map).

Steps to reproduce:
Any Label or RichTextLabel

[Calinou]

Fixed by #49377.