Improve consistency of inventory (and alike) imagery

In a number of places, background[] is misused to place the
inventory backdrop images.  Where appropriate, image[] is used
instead, so that "ui_form_bg.png" actually serves as the one
and only true background image.

In so doing, I was able to remake the bag inventory images,
making them only big as is actually needed to hold 1, 2, or 3
rows of inventory slots.

This, in turn, allows a standardized main inventory image to
occupy the lower part of the window, which allows for
consistent inventory image positioning and sizing from one
page to another.

I also removed ui_misc_form.png.  Nothing in UI uses it, and
any external mods that used it can just use the standard
inventory and its background.

Lastly, I reduced the background image to 512x384 px.  It was
unnecessarily large before, considering it has no real detail.

The larger inventory images are all 512px wide, and multiples
of 64px in height.  Before, they were oddly sized.

internal.lua

@@ -65,13 +65,13 @@ function unified_inventory.get_formspec(player, page)
 	local formspec = {
 		"size[14,10]",
 		pagedef.formspec_prepend and "" or "no_prepend[]",
-		"background[-0.19,-0.25;14.4,10.75;ui_form_bg.png]" -- Background
+		unified_inventory.standard_background -- Background
 	}
 	local n = 4
 
 	if draw_lite_mode then
 		formspec[1] = "size[11,7.7]"
-		formspec[3] = "background[-0.19,-0.2;11.4,8.4;ui_form_bg.png]"
+		formspec[3] = unified_inventory.standard_background
 	end
 
 	if unified_inventory.is_creative(player_name)
@@ -130,7 +130,7 @@ function unified_inventory.get_formspec(player, page)
 	if fsdata.draw_inventory ~= false then
 		-- Player inventory
 		formspec[n] = "listcolors[#00000000;#00000000]"
-		formspec[n+1] = "list[current_player;main;0,"..(ui_peruser.formspec_y + 3.5)..";8,4;]"
+		formspec[n+1] = string.gsub(unified_inventory.standard_inv, "YYY", ui_peruser.formspec_y + 3.5)
 		n = n+2
 	end