RHI: Add explicit RenderPass abstraction (BeginRenderPass/EndRenderPass)

New abstractions:
- RHIFramebuffer: Framebuffer interface with Initialize/Bind/GetHandle
- RHIRenderPass: RenderPass interface with AttachmentDesc for load/store actions
- D3D12Framebuffer/D3D12RenderPass: D3D12 implementation
- OpenGLFramebuffer/OpenGLRenderPass: OpenGL implementation (adapted from existing)

RHICommandList changes:
- Added BeginRenderPass(RHIRenderPass*, RHIFramebuffer*, Rect, clearValues...)
- Added EndRenderPass()

Implementation notes:
- D3D12: Uses OMSetRenderTargets + ClearRenderTargetView (fallback from native RenderPass API)
- OpenGL: Uses glBindFramebuffer + glClearBufferfv for LoadOp handling
- Old SetRenderTargets/ClearRenderTarget retained for backward compatibility

All 845 tests pass.

engine/src/RHI/OpenGL/OpenGLRenderPass.cpp

@@ -0,0 +1,38 @@
+#include "XCEngine/RHI/OpenGL/OpenGLRenderPass.h"
+
+namespace XCEngine {
+namespace RHI {
+
+OpenGLRenderPass::OpenGLRenderPass() {
+}
+
+OpenGLRenderPass::~OpenGLRenderPass() {
+    Shutdown();
+}
+
+void OpenGLRenderPass::Shutdown() {
+    m_colorAttachments.clear();
+    m_colorAttachmentCount = 0;
+    m_hasDepthStencil = false;
+}
+
+bool OpenGLRenderPass::Initialize(uint32_t colorAttachmentCount, const AttachmentDesc* colorAttachments,
+                                const AttachmentDesc* depthStencilAttachment) {
+    m_colorAttachmentCount = colorAttachmentCount;
+    m_colorAttachments.resize(colorAttachmentCount);
+    for (uint32_t i = 0; i < colorAttachmentCount; ++i) {
+        m_colorAttachments[i] = colorAttachments[i];
+    }
+
+    if (depthStencilAttachment != nullptr) {
+        m_hasDepthStencil = true;
+        m_depthStencilAttachment = *depthStencilAttachment;
+    } else {
+        m_hasDepthStencil = false;
+    }
+
+    return true;
+}
+
+} // namespace RHI
+} // namespace XCEngine