render-test: Add copy-source usage for render targets

I found that Slang-RHI/WGPU was not able to copy from render targets to staging buffers.

This helps to address issue shader-slang#4943.

tools/render-test/render-test-main.cpp

@@ -675,7 +675,7 @@ void RenderTestApp::_initializeRenderPass()
     colorBufferDesc.size.depth = 1;
     colorBufferDesc.mipLevelCount = 1;
     colorBufferDesc.format = Format::R8G8B8A8_UNORM;
-    colorBufferDesc.usage = TextureUsage::RenderTarget;
+    colorBufferDesc.usage = TextureUsage::RenderTarget | TextureUsage::CopySource;
     colorBufferDesc.defaultState = ResourceState::RenderTarget;
     m_colorBuffer = m_device->createTexture(colorBufferDesc, nullptr);
     SLANG_ASSERT(m_colorBuffer);