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/OpenGLDescriptorPool.cpp

@@ -0,0 +1,62 @@
+#include "XCEngine/RHI/OpenGL/OpenGLDescriptorPool.h"
+#include "XCEngine/RHI/OpenGL/OpenGLDescriptorSet.h"
+#include "XCEngine/RHI/OpenGL/OpenGLTextureUnitAllocator.h"
+
+namespace XCEngine {
+namespace RHI {
+
+OpenGLDescriptorPool::OpenGLDescriptorPool()
+    : m_type(DescriptorHeapType::CBV_SRV_UAV)
+    , m_maxSets(0)
+    , m_textureUnitAllocator(nullptr) {
+}
+
+OpenGLDescriptorPool::~OpenGLDescriptorPool() {
+    Shutdown();
+}
+
+bool OpenGLDescriptorPool::Initialize(const DescriptorPoolDesc& desc) {
+    m_type = desc.type;
+    m_maxSets = desc.descriptorCount;
+    return true;
+}
+
+void OpenGLDescriptorPool::Shutdown() {
+    for (auto* set : m_allocatedSets) {
+        delete set;
+    }
+    m_allocatedSets.clear();
+    m_textureUnitAllocator = nullptr;
+}
+
+RHIDescriptorSet* OpenGLDescriptorPool::AllocateSet(const DescriptorSetLayoutDesc& layout) {
+    if (m_allocatedSets.size() >= m_maxSets) {
+        return nullptr;
+    }
+
+    OpenGLDescriptorSet* newSet = new OpenGLDescriptorSet();
+    if (!newSet->Initialize(m_textureUnitAllocator, layout.bindingCount, layout)) {
+        delete newSet;
+        return nullptr;
+    }
+
+    m_allocatedSets.push_back(newSet);
+    return newSet;
+}
+
+void OpenGLDescriptorPool::FreeSet(RHIDescriptorSet* set) {
+    if (set == nullptr) {
+        return;
+    }
+
+    for (auto it = m_allocatedSets.begin(); it != m_allocatedSets.end(); ++it) {
+        if (*it == set) {
+            m_allocatedSets.erase(it);
+            delete set;
+            return;
+        }
+    }
+}
+
+} // namespace RHI
+} // namespace XCEngine