feat: allow deselecting selected layer by clicking 'select all' button

Fixes #476

Author: Bentroen

scripts/control_create/control_create.gml

@@ -307,6 +307,7 @@ function control_create() {
 	dragvol = 0
 	dragstereob = 0
 	dragstereo = 0
+	selected_layers = ds_list_create()
 
 	// Piano
 	show_piano = 1

scripts/control_draw/control_draw.gml

@@ -1398,11 +1398,18 @@ function control_draw() {
 			// Select all
 		    if (draw_layericon(2, x1 + 162 - !realvolume-realstereo * 10, y1 + 8, condstr(language != 1, "Select all note blocks in this layer\n(Hold Ctrl to select multiple layers)", "选择本层所有方块\n（按住Ctrl选择多层）"), 0, 0)) {
 		        playing = 0
-				if (!keyboard_check(vk_control)) {
-					selection_place(0)
+				
+				// TODO: replace with array_contains() if we ever upgrade GameMaker...
+				var layer_id = startb + b
+				var layer_selected = ds_list_find_index(selected_layers, layer_id) > -1
+				if (layer_selected) {
+					selection_remove(0, startb + b, enda, startb + b, 0, 0)
+					ds_list_delete_value(selected_layers, layer_id)
+				} else {
+					selection_add(0, startb + b, enda, startb + b, 0, 0)
+					ds_list_add(selected_layers, layer_id)
 				}
-		        selection_add(0, startb + b, enda, startb + b, 0, 0)
-		    }
+			}
 			// Add layer
 		    if (draw_layericon(3, x1 + 180 - !realvolume-realstereo * 10, y1 + 8, condstr(language != 1, "Add empty layer here", "新建层"), 0, 0)) {
 		        playing = 0

scripts/selection_code_update/selection_code_update.gml

@@ -3,7 +3,7 @@ function selection_code_update() {
 	// Updates the selection code
 	var a, b, ca, cb;
 
-	if (selected = 0) return 0
+	if (selected = 0) ds_list_clear(selected_layers) return 0
 	ca = 0
 
 	var _selection_l = selection_l

scripts/selection_place/selection_place.gml

@@ -1,6 +1,7 @@
 function selection_place(argument0) {
 	// selection_place(undo)
 	// Releases the selection.
+	ds_list_clear(selected_layers)
 	if (selected = 0) return 0
 	selection_remove(0, 0, 0, 0, 1, argument0)