RHI: Add DescriptorSet abstraction for D3D12 and OpenGL backends

- Add RHIDescriptorSet base class with Update/UpdateSampler/GetNativeHandle
- Add RHIDescriptorPool with AllocateSet/FreeSet methods
- Add SetGraphicsDescriptorSets/SetComputeDescriptorSets to RHICommandList
- Implement D3D12DescriptorSet using descriptor heap allocation
- Implement OpenGLDescriptorSet using TextureUnitAllocator
- Add CreateDescriptorPool/CreateDescriptorSet factory methods to RHIDevice
- Fix unit test SetVertexBuffer -> SetVertexBuffers API
- Add SetVertexBuffer convenience method for D3D12 backward compatibility
- Update CMakeLists.txt with new source files

engine/src/RHI/OpenGL/OpenGLDevice.cpp

@@ -18,6 +18,8 @@
 #include "XCEngine/RHI/OpenGL/OpenGLUniformBufferManager.h"
 #include "XCEngine/RHI/OpenGL/OpenGLFramebuffer.h"
 #include "XCEngine/RHI/OpenGL/OpenGLResourceView.h"
+#include "XCEngine/RHI/OpenGL/OpenGLDescriptorPool.h"
+#include "XCEngine/RHI/OpenGL/OpenGLDescriptorSet.h"
 #include "XCEngine/Debug/Logger.h"
 
 typedef const char* (WINAPI* PFNWGLGETEXTENSIONSSTRINGARBPROC)(HDC hdc);
@@ -384,6 +386,23 @@ RHISampler* OpenGLDevice::CreateSampler(const SamplerDesc& desc) {
     return sampler;
 }
 
+RHIDescriptorPool* OpenGLDevice::CreateDescriptorPool(const DescriptorPoolDesc& desc) {
+    auto* pool = new OpenGLDescriptorPool();
+    if (pool->Initialize(desc)) {
+        pool->SetTextureUnitAllocator(m_textureUnitAllocator.get());
+        return pool;
+    }
+    delete pool;
+    return nullptr;
+}
+
+RHIDescriptorSet* OpenGLDevice::CreateDescriptorSet(RHIDescriptorPool* pool, const DescriptorSetLayoutDesc& layout) {
+    if (pool == nullptr) {
+        return nullptr;
+    }
+    return pool->AllocateSet(layout);
+}
+
 RHIResourceView* OpenGLDevice::CreateRenderTargetView(RHITexture* texture, const ResourceViewDesc& desc) {
     if (!texture) {
         return nullptr;