libvnc: add ExtendedMouseButtons support

[marcofortina]

Summary

Adds ExtendedMouseButtons support for LibVNCServer/LibVNCClient.

The RFB ExtendedMouseButtons pseudo-encoding allows clients and servers to
negotiate extended PointerEvent messages with an extra button-state byte. This
is needed for mouse buttons beyond the legacy one-byte PointerEvent mask, such
as forward/back-capable devices and additional mouse buttons.

Changes

• Add rfbEncodingExtendedMouseButtons and named button masks up to button 15.
• Server side:
  • Accept rfbEncodingExtendedMouseButtons in SetEncodings.
  • Send the empty pseudo-rectangle acknowledgement required by the extension.
  • Track whether ExtendedMouseButtons has been negotiated for each client.
  • Decode extended PointerEvent messages only after the extension has been negotiated.
  • Preserve legacy PointerEvent handling when the extension is not negotiated.
• Client side:
  • Advertise rfbEncodingExtendedMouseButtons.
  • Track the server acknowledgement.
  • Send normal legacy PointerEvent messages when possible.
  • Send normal button 8 without requiring the extension.
  • Send buttons above button 8 with the extended PointerEvent form after negotiation.
  • Preserve legacy behavior when the server does not acknowledge the extension.
• Add regression coverage for:
  • ExtendedMouseButtons negotiation.
  • Server-side acknowledgement handling.
  • Client-side PointerEvent serialization.
  • Legacy fallback behavior when the extension is not negotiated.

Validation

Local validation:

git diff --check
cmake -S . -B build-686-patch \
  -DWITH_EXAMPLES=OFF \
  -DWITH_TESTS=ON \
  -DWITH_OPENSSL=OFF \
  -DWITH_GNUTLS=OFF \
  -DWITH_GCRYPT=OFF \
  -DWITH_SDL=OFF \
  -DWITH_GTK=OFF \
  -DWITH_QT=OFF \
  -DWITH_FFMPEG=OFF \
  -DWITH_XCB=OFF \
  -DWITH_LIBSSHTUNNEL=OFF \
  -DWITH_SYSTEMD=OFF \
  -DCMAKE_BUILD_TYPE=Debug
cmake --build build-686-patch --parallel 1
ctest --test-dir build-686-patch --output-on-failure

Result:

100% tests passed, 0 tests failed out of 6

Notes

This implements the negotiated ExtendedMouseButtons protocol path without
changing the legacy PointerEvent behavior for peers that do not negotiate the
extension.

This PR does not add any LibVNC-specific wire protocol. It uses the standard
RFB ExtendedMouseButtons pseudo-encoding and keeps the feature opt-in through
normal SetEncodings negotiation.

Closes #686.