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/D3D12/D3D12Device.cpp

@@ -4,6 +4,7 @@
 #include "XCEngine/RHI/D3D12/D3D12CommandAllocator.h"
 #include "XCEngine/RHI/D3D12/D3D12Fence.h"
 #include "XCEngine/RHI/D3D12/D3D12DescriptorHeap.h"
+#include "XCEngine/RHI/D3D12/D3D12DescriptorSet.h"
 #include "XCEngine/RHI/D3D12/D3D12QueryHeap.h"
 #include "XCEngine/RHI/D3D12/D3D12RootSignature.h"
 #include "XCEngine/RHI/D3D12/D3D12PipelineLayout.h"
@@ -324,6 +325,24 @@ RHISampler* D3D12Device::CreateSampler(const SamplerDesc& desc) {
     return nullptr;
 }
 
+RHIDescriptorPool* D3D12Device::CreateDescriptorPool(const DescriptorPoolDesc& desc) {
+    auto* pool = new D3D12DescriptorHeap();
+    DescriptorPoolDesc poolDesc = desc;
+    poolDesc.device = m_device.Get();
+    if (pool->Initialize(poolDesc)) {
+        return pool;
+    }
+    delete pool;
+    return nullptr;
+}
+
+RHIDescriptorSet* D3D12Device::CreateDescriptorSet(RHIDescriptorPool* pool, const DescriptorSetLayoutDesc& layout) {
+    if (pool == nullptr) {
+        return nullptr;
+    }
+    return pool->AllocateSet(layout);
+}
+
 RHIFence* D3D12Device::CreateFence(const FenceDesc& desc) {
     auto* fence = new D3D12Fence();
     if (fence->Initialize(m_device.Get(), desc.initialValue)) {