use color alpha in button backgrounds

Dependencies

• [2] 54HEXCWV clearer API for drawing a button
• [3] PP2IIHL6 stop putting button state in a global
• [4] XX7G2FFJ intermingle freehand line drawings with text

Change contents

• replacement in button.lua at line 15
  [3.296]→[2.415:479](∅→∅)
  − love.graphics.setColor(params.bg.r, params.bg.g, params.bg.b)
  [3.296]

  [3.2230]
  + love.graphics.setColor(params.bg.r, params.bg.g, params.bg.b, params.bg.a)