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.

tests/gtest/plugin_ofx_integration_test.cpp

@@ -119,7 +119,7 @@ TEST(PluginIntegration, ChromaKeyerCreateAndRender)
 	olive::plugin::PluginJob job(instance, nullptr, row);
 	olive::TexturePtr output = std::make_shared<olive::Texture>(params);
 
-	olive::plugin::PluginRenderer renderer;
+	olive::plugin::PluginRenderer renderer(nullptr);
 	renderer.RenderPlugin(input, job, output, params, true, false);
 
 	EXPECT_TRUE(output->frame());