With View > Pixel Preview on, the tools still snapped to Smart Guides
and showed their guides; only the Object Highlighting outline was
hidden. Work with Smart Guides says Smart Guides cannot be used while
Snap to Grid or Pixel Preview is on.
ViewInfo has a pixel_preview field, and Session::with_tool_cx turns
Smart Guides (and Snap to Glyph, which needs them) off in the tools'
context while it is set. VectorcraftApp::view_info passes View > Pixel
Preview. View > Smart Guides keeps its setting.
Docs: docs/target-app-parity.md.
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