Fix RichTextLabel bottom margin for text clipping

[DictyosteliumDiscoideum]

When a style with a bottom margin was added to a rich text label node,
the bottom margin would be added to the text but the text would not be
clipped.
This adds a line to subtract the bottom margin from the label height
before checking another line should be drawn.
Fixes #87537. Does not fix the additional issue raised regarding
the right margins.

[Calinou]

Tested locally (rebased on top of master 0e9caa2), it works as expected after fixing the compilation error mentioned in my above review comments.

Before

rtl_bottom_margin_before.mp4

After

rtl_bottom_margin_after.mp4

[bruvzg]

Seems fine. There's also the same check for the autowrapped lines, and likely need the same change:

godot/scene/gui/rich_text_label.cpp

Lines 820 to 827 in da5f398

	Size2 ctrl_size = get_size();
	// Draw text.
	for (int line = 0; line < l.text_buf->get_line_count(); line++) {
	if (line > 0) {
	off.y += theme_cache.line_separation;
	}
	if (p_ofs.y + off.y >= ctrl_size.height) {

[akien-mga]

Thanks! And congrats for your first merged Godot contribution 🎉