Image - add drawing of rect

[flamendless]

Hi, it would be useful if drawing a rectangle is allowed in the Image API.

This would be useful for tilemap/spritesheet drawing and having an indicator (rect) in the current frame/index. Usage would be:

local spritesheet = love.graphics.newImage("player_walk_sheet.png")
Slab.Image("Spritesheet", {
  --basic drawing of the spritesheet
  Image = spritesheet,
  SubX = 0,
  SubY = 0,
  SubW = spritesheet:getWidth(),
  SubH = spritesheet:getHeight(),

  --add rectangle
  RectColor = {1, 0, 0, 1}, --rect border color as love.graphics.setColor(1, 0, 0, 1)
  RectLineWidth = 2, --same as love.graphics.setLineWidth(2)
  RectX = x_offset * frame_width,
  RectY = y_offset * frame_height,
  RectW = frame_width,
  RectH = frame_height,
})