libvncclient: guard SetDesktopSize support

[marcofortina]

Summary

Prevent libvncclient from sending SetDesktopSize to servers that have not advertised support for that client-to-server message.

Some servers close the connection when they receive the unsupported SetDesktopSize message type. SendExtDesktopSize() now follows the existing supported-message guard pattern used by other client-to-server send helpers.

Changes

• Make SendExtDesktopSize() return without writing when rfbSetDesktopSize is not supported by the server.
• Mark rfbSetDesktopSize as supported when an ExtendedDesktopSize update is received, because servers that support ExtendedDesktopSize must support SetDesktopSize as well.
• Add a small client capability regression test for the unsupported path.

Validation

cmake -S . -B build-587-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-587-patch --parallel 1
ctest --test-dir build-587-patch --output-on-failure

Result:

100% tests passed, 0 tests failed out of 6

Notes

Closes #587.