Add DesktopSize / ExtendedDesktopSize pseudo-encoding support

[rsp2k]

The RFB pseudo-encodings -223 (DesktopSize) and -308 (ExtendedDesktopSize) carry server-side geometry change notifications. asyncvnc2 currently has no parser for them, so when a server pushes one (TigerVNC does this on every xrandr resize, for example), Enc(...) raises ValueError and the connection drops on the next byte read.

What changed

• Enc gains DESKTOP_SIZE = -223 and EXTENDED_DESKTOP_SIZE = -308.
• The SetEncodings writer uses signed=True so negative pseudo-encodings serialize correctly when callers opt them in via EncList.
• Video.read converts the unsigned wire integer back to signed before the Enc(...) lookup. Existing raise-on-unknown-encoding behavior is preserved (silently skipping an unknown rect would desync the stream).
• Two new dispatch branches drain the pseudo-encoding payload and update Video.width/height through a small _handle_desktop_resize helper that only invalidates the framebuffer when the size actually changes (servers commonly echo the current size on connect).
• Client.screenshot re-requests on incomplete updates so callers don't deadlock when an unsolicited pseudo-encoding rect arrives mid-screenshot.
• Enc.default() now excludes pseudo-encodings (filter value > 0) with a docstring explaining the TigerVNC subscription quirk — see below.
• tests/test_desktop_size.py adds 5 unit tests covering _handle_desktop_resize no-op vs invalidate semantics and Video.read parsing of DESKTOP_SIZE plus EDS rects (including multi-screen payloads).

TigerVNC subscription quirk

If you advertise ExtendedDesktopSize in SetEncodings, the TigerVNC build in accetto/ubuntu-vnc-xfce-g3 returns EDS-only FramebufferUpdate responses (cnt=1, no pixel data) to every FramebufferUpdateRequest, even with incremental=False. This effectively starves the screenshot path. The docstring on Enc.default() warns about this so users who want subscription-style resize notifications opt in deliberately: EncList() + Enc.EXTENDED_DESKTOP_SIZE.

Verification

Tested against accetto/ubuntu-vnc-xfce-g3 and the existing pytest suite. Default-behavior screenshots and the new resize handling both work; the test suite passes unchanged.