img: remove padding_x from lab_img_render()

img: remove padding_x from lab_img_render()

padding_x in lab_img_render() was just to make sure window icons in the
titlebar doesn't get too close to adjacent buttons and it didn't felt
clean. To remove it from lab_img, this commit changes the size of window
icon buffers from (window.button.width)x(window.button.height) to
(window.button.width * 0.8)x(window.button.height) and horizontally
slide it by (window.button.width * 0.1) to keep the horizontal padding.

Note that the size of the hitbox for a window icon is kept the same and
nothing is changed from user's perspective.