Run apps on the desktop: SDK emulator, plus two fixes it exposed

[ihrfv]

Stacked on top of simmsb#24 (simmsb/inkview-rs), whose GetCanvas() fallback this branch needs — the base
here is emulator-getcanvas-fallback rather than master, so this shows only the five commits it
adds. It should land after simmsb#24 does.

What this adds

A desktop emulator. Some PocketBook SDK archives ship a host x86_64 build of libinkview.so
alongside the ARM one. Because inkview dlopens its library instead of linking it, building for
x86_64-unknown-linux-gnu and putting that directory on the loader path runs the real app on a
developer machine, in an X11 window:

just fetch-emu-assets                              # one-time, ~1.3 GB download, ~40 MB kept
just emu-image                                     # one-time, builds the container
just build-emu-app inkview-slint-demo
just pb_emu_resmode=3 run-emu inkview-slint-demo   # a window
just screenshot-emu inkview-slint-demo             # headless -> target/emu/screenshot.png

screenshot-emu runs against a throwaway Xvfb, so it works headlessly and in CI — the first
rendering check this repo has had.

Two fixes the emulator exposed, both independent of the tooling and revertible on their own:

• Screen::new now takes the panel down on the GetCanvas() path. Once the panel is active that
  canvas starts PanelHeightFBOffset() rows into the framebuffer (75 at 828x1200) while still
  reporting the full height, so everything drawn landed that far down and the last rows wrapped to
  the top. It only shows up when drawing happens after the Init handler — i.e. from the worker
  thread inkview-slint apps use — which is why it went unnoticed.
• inkview-slint announces the scale factor before setting the window size. set_size resolves its
  argument against the window's current scale, so passing a logical size first made slint record it
  as the physical one: an 828x1200 screen came out as a 331x480 window. Rendering was unaffected;
  what Window::size() reported was not.

Notes for review

• The host library is a GTK2/X11 binary built against Debian 9, so it runs in a debian:stretch-slim
  container (utils/emulator/Dockerfile) and binaries target glibc 2.24. utils/emulator/README.md
  covers when to reach for the firmware-level emulator instead.
• Only the 6.8 archive on the flxzt mirror ships the host subtree; utils/emulator/peek-sdk-archive.py
  checks another release by reading the 7z index over HTTP range requests, without downloading it.
• On macOS run-emu sets XQuartz up itself, including re-granting xhost on every run — XQuartz
  quits with its last client and comes back with an empty access list.
• Verified by running: the slint demo renders correctly under the emulator at several geometries,
  takes touch input, and each commit here builds on its own.

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