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/common/ffmpegutils.h

@@ -0,0 +1,89 @@
+/***
+
+  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_FFMPEGABSTRACTION_H
+#define OAK_FFMPEGABSTRACTION_H
+
+#include <ffmpeg_bridge/ffmpeg_bridge.h>
+
+#include <olive/core/core.h>
+
+#include "render/videoparams.h"
+
+namespace olive
+{
+
+using namespace core;
+
+/**
+ * @brief C++ adapter mapping Olive's native enums to bridge pixel/sample
+ * formats
+ *
+ * All "FFmpeg" formats here are actually the opaque FBPixelFormat /
+ * FBSampleFormat constants of the ffmpeg_bridge library; no FFmpeg header
+ * or structure is ever seen by the editor.
+ */
+class FFmpegUtils {
+public:
+	/**
+   * @brief Returns a bridge pixel format that can be used to convert a frame to a data type Olive supports with minimal data loss
+   *
+   * Named distinctly from the native PixelFormat overload below: with both
+   * taking a single argument, an unscoped enum argument would silently
+   * prefer an int overload over the PixelFormat one.
+   */
+	static int get_compatible_bridge_pixel_format(
+		int pix_fmt, PixelFormat maximum = PixelFormat::invalid);
+
+	/**
+   * @brief Returns a native pixel format that can be used to convert from a native frame to a bridge frame with minimal data loss
+   */
+	static PixelFormat get_compatible_pixel_format(const PixelFormat &pix_fmt);
+
+	/**
+   * @brief Returns a bridge pixel format for a given native pixel format
+   */
+	static int get_f_fmpeg_pixel_format(const PixelFormat &pix_fmt,
+									int channel_layout);
+
+	/**
+   * @brief Returns a native sample format type for a given bridge sample format
+   */
+	static SampleFormat get_native_sample_format(int smp_fmt);
+
+	/**
+   * @brief Returns a bridge sample format type for a given native type
+   */
+	static int get_f_fmpeg_sample_format(const SampleFormat &smp_fmt);
+
+	/**
+   * @brief Convert "JPEG"/full-range colorspace to its regular counterpart
+   *
+   * "JPEG "spaces are deprecated in favor of the regular space and setting `color_range`. For the
+   * time being, FFmpeg still uses these JPEG spaces, so for simplicity (since we *are* color_range
+   * aware), we use this function.
+   */
+	static int convert_jpeg_space_to_regular_space(int f);
+};
+
+}
+
+#endif // OAK_FFMPEGABSTRACTION_H