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/render/framemanager.cpp

@@ -0,0 +1,132 @@
+/***
+
+  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 "framemanager.h"
+
+#include <QDateTime>
+#include <QDebug>
+
+namespace olive
+{
+
+FrameManager *FrameManager::instance_ = nullptr;
+const int FrameManager::k_frame_lifetime = 5000;
+
+void FrameManager::create_instance()
+{
+	instance_ = new FrameManager();
+}
+
+void FrameManager::destroy_instance()
+{
+	delete instance_;
+	instance_ = nullptr;
+}
+
+FrameManager *FrameManager::instance()
+{
+	return instance_;
+}
+
+char *FrameManager::allocate(int size)
+{
+	if (instance()) {
+		return instance()->allocate_from_pool(size);
+	} else {
+		return new char[size];
+	}
+}
+
+void FrameManager::deallocate(int size, char *buffer)
+{
+	if (instance()) {
+		instance()->deallocate_to_pool(size, buffer);
+	} else {
+		delete[] buffer;
+	}
+}
+
+FrameManager::FrameManager()
+{
+	clear_timer_.setInterval(k_frame_lifetime);
+	connect(&clear_timer_, &QTimer::timeout, this,
+			&FrameManager::garbage_collection);
+	clear_timer_.start();
+}
+
+char *FrameManager::allocate_from_pool(int size)
+{
+	QMutexLocker locker(&mutex_);
+
+	std::list<Buffer> &buffer_list = pool_[size];
+	char *buf = nullptr;
+
+	if (buffer_list.empty()) {
+		buf = new char[size];
+	} else {
+		// Take this buffer from the list
+		buf = buffer_list.front().data;
+		buffer_list.pop_front();
+	}
+
+	return buf;
+}
+
+void FrameManager::deallocate_to_pool(int size, char *buffer)
+{
+	QMutexLocker locker(&mutex_);
+
+	std::list<Buffer> &buffer_list = pool_[size];
+
+	buffer_list.push_back({ QDateTime::currentMSecsSinceEpoch(), buffer });
+}
+
+void FrameManager::garbage_collection()
+{
+	QMutexLocker locker(&mutex_);
+
+	qint64 min_life = QDateTime::currentMSecsSinceEpoch() - k_frame_lifetime;
+
+	for (auto it = pool_.begin(); it != pool_.end(); it++) {
+		std::list<Buffer> &list = it->second;
+
+		while (list.size() > 0 && list.front().time < min_life) {
+			delete[] list.front().data;
+			list.pop_front();
+		}
+	}
+}
+
+FrameManager::~FrameManager()
+{
+	QMutexLocker locker(&mutex_);
+
+	for (auto it = pool_.begin(); it != pool_.end(); it++) {
+		std::list<Buffer> &list = it->second;
+		for (auto jt = list.begin(); jt != list.end(); jt++) {
+			delete[] (*jt).data;
+		}
+	}
+
+	pool_.clear();
+}
+
+}