fix: overlay padding uses one value for every side #207

What

An overlay's padding is one value (pad.l is used for every side), so a HUD that wants different horizontal and vertical padding cannot express it through the style. cellar's readout had 12 px horizontal and 8 px vertical padding before moving onto the flow layout, and now uses 8 px on every side (octalide/cellar#167).

Acceptance

• Overlay padding reads all four sides from the style's pad, as windows do
• Existing themes keep their current look