Autoslice could cause slicing to start before init done.

This could cause slicing to hang forever until settings where changed. So stop too-early slicing, but keep a memo of wether we've already been requested to slice in the initialization interval, so auto-slice doesn't fail either.

CURA-11815

plugins/CuraEngineBackend/CuraEngineBackend.py

@@ -68,6 +68,9 @@ def __init__(self) -> None:
         """
 
         super().__init__()
+        self._init_done = False
+        self._immediate_slice_after_init = False
+
         # Find out where the engine is located, and how it is called.
         # This depends on how Cura is packaged and which OS we are running on.
         executable_name = "CuraEngine"
@@ -268,6 +271,10 @@ def initialize(self) -> None:
         self._machine_error_checker = application.getMachineErrorChecker()
         self._machine_error_checker.errorCheckFinished.connect(self._onStackErrorCheckFinished)
 
+        self._init_done = True
+        if self._immediate_slice_after_init:
+            self.slice()
+
     def close(self) -> None:
         """Terminate the engine process.
 
@@ -342,6 +349,11 @@ def getLatestSnapshot(self) -> Optional[QImage]:
     def slice(self) -> None:
         """Perform a slice of the scene."""
 
+        if not self._init_done:
+            self._immediate_slice_after_init = True
+            return
+        self._immediate_slice_after_init = False
+
         self._createSnapshot()
 
         self.startPlugins()