Return StylePainter::paint_button() before color initializations
when possible and replace an overdrawn rect in paint_button_new()
with more explicit shadow lines. Also standardize some comments.
This adds a "shine" effect to the bottom-right edges of pressed
and checked buttons, complementing the sunken shadow already
present at top-left and creating greater illusion of depth.
It is similar to the effect used in classic Windows and is
theme agnostic in Serenity, but produces the best result when
ThreedHighlight is a radiant color of Button.
