One glyph of prepared text: where it goes, the part of the glyph atlas it shows and its tint.
begin_prepare_text through end_prepare_text fill Dynamic_Font.current_quads with one of these per glyph, placed with the
pen at the origin. ul is the corner with the smaller y and lr the opposite one; uv0 is the atlas coordinate drawn at
ul and uv1 the one drawn at lr. p0 to p3 are the four corners the same box is drawn with, and u0, v0, u1, v1 the
atlas box before it is flipped to match, for code that wants those forms.