X11: fix ICCCM selections, add INCR and alpha, honor Xft.dpi

Clipboard and primary selection:
- Take ownership at the time of the latest key or button event instead of
  CurrentTime; TIMESTAMP answers with the time each selection was taken.
- Forget the stored text on SelectionClear.
- Answer MULTIPLE (failed pairs get None) and refuse selections other than
  CLIPBOARD and PRIMARY.
- Send payloads above the request limit with INCR instead of refusing them,
  with a per-chunk timeout and cleanup on display close.
- Receive INCR transfers: delete the marker to start, append chunks on
  PropertyNewValue, and finish on a zero-length chunk.

Translucency:
- CPU toplevels and popups use a 32-bit TrueColor visual with their own
  colormap when the server has one. Shared-memory and plain images use the
  window's visual and depth.

HiDPI:
- Read Xft.dpi at display open and derive the scale, clamped to 1x-4x.
  Logical creation sizes and set_size requests become device pixels, and
  reported geometry carries the scale.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

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