Function background_rect 

pub fn background_rect(
    text_x: u32,
    text_y: u32,
    text_width_px: u32,
    font_height: u32,
    padding: u32,
) -> (u32, u32, u32, u32)

Render a background box behind the overlay text.

Returns (x, y, width, height) of the background rectangle with padding.