build: split the engine into liboakengine.so; worker drops the UI entirely

Physical split: app/{audio,cli,codec,common,config,node,pluginSupport,
render,task,timeline,undo,tool,shaders} plus coreengine, version and
ui/icons+colorcoding move to a new top-level engine/ tree, built as
liboakengine.so (shared). The render backends (oakgl/oakvulkan) move
with it and link the engine library instead of embedding a static
render-core subset (libolive-rendercore is gone).

- oak-render-worker now links liboakengine instead of the whole
  libolive-editor object set: 336MB -> 2.9MB, no Qt Widgets UI
- the editor links liboakengine for the engine and keeps only UI
  objects in libolive-editor
- install/packaging: GNUInstallDirs libdir on Linux, bundle copy on
  macOS, oakengine.dll staged for NSIS, AppImage validation entry
- fix backend lookup for the new layout: DynamicRenderer searched
  ../app but backends now live in engine/; a stale pre-split liboakgl
  in the build tree got dlopened instead, re-initialized and later
  destroyed the interposed engine statics (full-suite segfault at
  DialogSequenceParameterTab, found via gdb watchpoint)

engine/task/customcache/CMakeLists.txt

@@ -0,0 +1,22 @@
+# Olive - Non-Linear Video Editor
+# Copyright (C) 2022 Olive Team
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+set(OLIVE_SOURCES
+        ${OLIVE_SOURCES}
+        task/customcache/customcachetask.cpp
+        task/customcache/customcachetask.h
+        PARENT_SCOPE
+)

engine/task/customcache/customcachetask.cpp

@@ -0,0 +1,64 @@
+/***
+
+  Olive - Non-Linear Video Editor
+  Copyright (C) 2022 Olive Team
+  Modifications Copyright (C) 2025 mikesolar
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#include "customcachetask.h"
+
+namespace olive
+{
+
+CustomCacheTask::CustomCacheTask(const QString &sequence_name)
+	: cancelled_through_finish_(false)
+{
+	set_title(tr("Caching custom range for \"%1\"").arg(sequence_name));
+}
+
+void CustomCacheTask::finish()
+{
+	mutex_.lock();
+
+	cancelled_through_finish_ = true;
+	Cancel();
+
+	mutex_.unlock();
+}
+
+bool CustomCacheTask::run()
+{
+	mutex_.lock();
+
+	while (!is_cancelled()) {
+		wait_cond_.wait(&mutex_);
+	}
+
+	mutex_.unlock();
+
+	return true;
+}
+
+void CustomCacheTask::CancelEvent()
+{
+	if (!cancelled_through_finish_) {
+		emit cancelled();
+	}
+	wait_cond_.wakeOne();
+}
+
+}

engine/task/customcache/customcachetask.h

@@ -0,0 +1,58 @@
+/***
+
+  Olive - Non-Linear Video Editor
+  Copyright (C) 2022 Olive Team
+  Modifications Copyright (C) 2025 mikesolar
+
+  This program is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  This program is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+***/
+
+#ifndef OAK_CUSTOMCACHETASK_H
+#define OAK_CUSTOMCACHETASK_H
+
+#include <QMutex>
+#include <QWaitCondition>
+
+#include "task/task.h"
+
+namespace olive
+{
+
+class CustomCacheTask : public Task {
+	Q_OBJECT
+public:
+	CustomCacheTask(const QString &sequence_name);
+
+	void finish();
+
+signals:
+	void cancelled();
+
+protected:
+	virtual bool run() override;
+
+	virtual void CancelEvent() override;
+
+private:
+	QMutex mutex_;
+
+	QWaitCondition wait_cond_;
+
+	bool cancelled_through_finish_;
+};
+
+}
+
+#endif // OAK_CUSTOMCACHETASK_H