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/export/export.h

@@ -0,0 +1,84 @@
+/***
+
+  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_EXPORTTASK_H
+#define OAK_EXPORTTASK_H
+
+#include "codec/encoder.h"
+#include "node/output/viewer/viewer.h"
+#include "render/colorprocessor.h"
+#include "render/projectcopier.h"
+#include "task/render/render.h"
+#include "task/task.h"
+
+namespace olive
+{
+
+class ExportTask : public RenderTask {
+	Q_OBJECT
+public:
+	ExportTask(ViewerOutput *viewer_node, ColorManager *color_manager,
+			   const EncodingParams &params);
+
+protected:
+	virtual bool run() override;
+
+	virtual bool frame_downloaded(FramePtr frame, const Rational &time) override;
+
+	virtual bool audio_downloaded(const TimeRange &range,
+								 const SampleBuffer &samples) override;
+
+	virtual bool encode_subtitle(const SubtitleBlock *sub) override;
+
+	virtual bool two_step_frame_rendering() const override
+	{
+		return false;
+	}
+
+private:
+	bool write_audio_loop(const TimeRange &time, const SampleBuffer &samples);
+
+	ProjectCopier *copier_;
+
+	QHash<Rational, FramePtr> time_map_;
+
+	QHash<TimeRange, SampleBuffer> audio_map_;
+
+	ColorManager *color_manager_;
+
+	EncodingParams params_;
+
+	std::shared_ptr<Encoder> encoder_;
+
+	std::shared_ptr<Encoder> subtitle_encoder_;
+
+	ColorProcessorPtr color_processor_;
+
+	int64_t frame_time_;
+
+	Rational audio_time_;
+
+	TimeRange export_range_;
+};
+
+}
+
+#endif // OAK_EXPORTTASK_H