feat(render): dynamic OpenGL/Vulkan backend split and backend-neutral viewer

- Extract libolive-rendercore static library to minimize backend link boundary.
- Add DynamicRenderer adapter with C ABI (oakgl/oakvulkan shared libs).
- Make OAK_ENABLE_DYNAMIC_RENDER_BACKEND default ON with OpenGL fallback.
- Implement VulkanRenderer prototype (textures, shaders, UBO blit, readback).
- Add backend-neutral viewer readback path (offscreen -> QImage -> QPainter).
- Refactor PluginRenderer to be renderer-agnostic; OFX plugins fall back to CPU
  path on non-OpenGL backends while preserving OpenGL render path.
- Add Renderer::AttachOutputTexture/DetachOutputTexture and C ABI forwards.
- Update docs/zh/render-backend-dynamic-plan.md for Phase 3/4/5.

app/render/opengl/openglrenderer.cpp

@@ -27,7 +27,9 @@
 #include <QOpenGLExtraFunctions>
 #include <QRegularExpression>
 
+#include "common/filefunctions.h"
 #include "config/config.h"
+#include "render/job/shaderjob.h"
 
 namespace olive
 {
@@ -406,6 +408,18 @@ void OpenGLRenderer::Flush()
 	}
 }
 
+void OpenGLRenderer::AttachOutputTexture(olive::Texture *texture)
+{
+	if (texture) {
+		AttachTextureAsDestination(texture->id());
+	}
+}
+
+void OpenGLRenderer::DetachOutputTexture()
+{
+	DetachTextureAsDestination();
+}
+
 Color OpenGLRenderer::GetPixelFromTexture(Texture *texture, const QPointF &pt)
 {
 	AttachTextureAsDestination(texture->id());