Computer dark visibility tweaks (#1829)

changes:

Holo-computers' displays now draw above the lighting layer, allowing them to be seen in the dark.
APCs' displays now draw above the lighting layer, allowing them to be seen in the dark.

code/modules/modular_computers/computers/modular_computer/variables.dm

@@ -35,6 +35,8 @@
 	var/steel_sheet_cost = 5								// Amount of steel sheets refunded when disassembling an empty frame of this computer.
 	var/light_strength = 0									// Intensity of light this computer emits. Comparable to numbers light fixtures use.
 	var/list/idle_threads = list()							// Idle programs on background. They still receive process calls but can't be interacted with.
+	var/power_has_failed = FALSE
+	var/is_holographic = FALSE
 
 	// Damage of the chassis. If the chassis takes too much damage it will break apart.
 	var/damage = 0				// Current damage level