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/D3D12DescriptorSet.cpp

@@ -0,0 +1,69 @@
+#include "XCEngine/RHI/D3D12/D3D12DescriptorSet.h"
+#include "XCEngine/RHI/D3D12/D3D12DescriptorHeap.h"
+
+namespace XCEngine {
+namespace RHI {
+
+D3D12DescriptorSet::D3D12DescriptorSet()
+    : m_heap(nullptr)
+    , m_offset(0)
+    , m_count(0)
+    , m_bindingCount(0)
+    , m_bindings(nullptr) {
+}
+
+D3D12DescriptorSet::~D3D12DescriptorSet() {
+    Shutdown();
+}
+
+bool D3D12DescriptorSet::Initialize(D3D12DescriptorHeap* heap, uint32_t offset, uint32_t count, const DescriptorSetLayoutDesc& layout) {
+    m_heap = heap;
+    m_offset = offset;
+    m_count = count;
+    m_bindingCount = layout.bindingCount;
+    
+    if (layout.bindingCount > 0 && layout.bindings != nullptr) {
+        m_bindings = new DescriptorSetLayoutBinding[layout.bindingCount];
+        for (uint32_t i = 0; i < layout.bindingCount; ++i) {
+            m_bindings[i] = layout.bindings[i];
+        }
+    }
+    
+    return true;
+}
+
+void D3D12DescriptorSet::Shutdown() {
+    m_heap = nullptr;
+    m_offset = 0;
+    m_count = 0;
+    m_bindingCount = 0;
+    if (m_bindings != nullptr) {
+        delete[] m_bindings;
+        m_bindings = nullptr;
+    }
+}
+
+void D3D12DescriptorSet::Update(uint32_t offset, RHIResourceView* view) {
+    (void)offset;
+    (void)view;
+}
+
+void D3D12DescriptorSet::UpdateSampler(uint32_t offset, RHISampler* sampler) {
+    (void)offset;
+    (void)sampler;
+}
+
+void* D3D12DescriptorSet::GetNativeHandle() {
+    return this;
+}
+
+D3D12_GPU_DESCRIPTOR_HANDLE D3D12DescriptorSet::GetGPUHandle(uint32_t index) const {
+    if (m_heap == nullptr) {
+        return D3D12_GPU_DESCRIPTOR_HANDLE{0};
+    }
+    GPUDescriptorHandle handle = m_heap->GetGPUDescriptorHandle(m_offset + index);
+    return D3D12_GPU_DESCRIPTOR_HANDLE{ handle.ptr };
+}
+
+} // namespace RHI
+} // namespace XCEngine