examples/client: fix SDLvncviewer numpad keys

[marcofortina]

Summary

Fix SDLvncviewer keypad handling so keypad digits are not sent twice with NumLock enabled, and keypad keys can be translated to navigation keysyms when NumLock is disabled.

Issue #315 reports two Windows/MinGW/SDL2 problems:

• with NumLock ON, keypad digits can be sent twice because SDL_KEYDOWN sends an XK_KP_* keysym and SDL_TEXTINPUT then sends the printable character;
• with NumLock OFF, keypad keys should behave as keypad navigation keys instead of printable digits.

Changes

• Track NumLock state inside SDLvncviewer, initialized from SDL_GetModState().
• Update the tracked state when SDL reports SDLK_NUMLOCKCLEAR.
• Map keypad digits to navigation keypad keysyms when tracked NumLock is off.
• Suppress the matching one-character SDL_TEXTINPUT event after keypad keydown, avoiding duplicate keypad text.
• Avoid sending key events for unmapped SDL keysyms.

Validation

Local Linux/no-SDL regression build:

cmake -S . -B build-315-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-315-patch --parallel 1
ctest --test-dir build-315-patch --output-on-failure

Result:

100% tests passed, 0 tests failed out of 5

Windows/MSYS2 UCRT64/SDL2 smoke

Validated with:

• examples/server/vncev.exe
• examples/client/SDLvncviewer.exe localhost
• Windows On-Screen Keyboard numeric keypad

Result:

• NumLock ON: keypad digits generated only keypad keysyms, with no duplicate ASCII text events.
• NumLock OFF: keypad keys generated keypad navigation keysyms instead of numeric ASCII text.

Notes

Closes #315.