Ports table item autoalign from Bay (#6946)

Made it so items get their pixel x/y reset on pickup.
    Thrown items now also get their pixel placement slightly randomized.
    Tweaked the center of mass on a boatload of items to be more accurate to their sprite art.
    Replaced a bunch of randomized pixel placement code into a unifying proc; randpixel_xy() that uses an item's randpixel var.

code/modules/materials/material_sheets.dm

@@ -6,6 +6,7 @@
 	throw_speed = 3
 	throw_range = 3
 	max_amount = 50
+	center_of_mass = null
 
 	var/default_type = DEFAULT_WALL_MATERIAL
 	var/material/material
@@ -15,8 +16,7 @@
 
 /obj/item/stack/material/Initialize()
 	. = ..()
-	pixel_x = rand(0,4)-4
-	pixel_y = rand(0,4)-4
+	randpixel_xy()
 
 	if(!default_type)
 		default_type = DEFAULT_WALL_MATERIAL