Split retained legacy engine target

cmake/PanoPainterSources.cmake

@@ -1,4 +1,4 @@
-set(PP_LEGACY_APP_SOURCES
+set(PP_LEGACY_ENGINE_SOURCES
     src/abr.cpp
     src/action.cpp
     src/asset.cpp
@@ -8,14 +8,25 @@ set(PP_LEGACY_APP_SOURCES
     src/canvas.cpp
     src/canvas_actions.cpp
     src/canvas_layer.cpp
-    src/canvas_modes.cpp
     src/event.cpp
     src/font.cpp
     src/hmd.cpp
     src/image.cpp
-    src/layout.cpp
     src/log.cpp
     src/mp4enc.cpp
+    src/rtt.cpp
+    src/serializer.cpp
+    src/settings.cpp
+    src/shader.cpp
+    src/shape.cpp
+    src/texture.cpp
+    src/util.cpp
+    src/wacom.cpp
+)
+
+set(PP_LEGACY_APP_SOURCES
+    src/canvas_modes.cpp
+    src/layout.cpp
     src/node.cpp
     src/node_border.cpp
     src/node_button.cpp
@@ -36,14 +47,6 @@ set(PP_LEGACY_APP_SOURCES
     src/node_text.cpp
     src/node_text_input.cpp
     src/pch.cpp
-    src/rtt.cpp
-    src/serializer.cpp
-    src/settings.cpp
-    src/shader.cpp
-    src/shape.cpp
-    src/texture.cpp
-    src/util.cpp
-    src/wacom.cpp
 )
 
 set(PP_PANOPAINTER_APP_SOURCES