libvncserver: read HTTP files in aligned chunks

[marcofortina]

Summary

Fixes the HTTP file transfer chunk size described in #199.

The HTTP server used BUF_SIZE - 1 for every file read so that .vnc files could be NUL-terminated before variable substitution. For non-.vnc assets this produced 32767-byte chunks even though the static HTTP buffer is 32768 bytes.

On strict-alignment ARM targets this can make the HTTP/WebSocket write path hit unaligned buffer handling when large files are sent in repeated chunks.

Changes

• Keep BUF_SIZE - 1 only for .vnc files that need NUL termination before substitutions.
• Use the full BUF_SIZE for all other HTTP files, preserving the aligned 32768-byte chunk size.
• Leave .vnc substitution behavior unchanged.

Validation

Tested with a minimal local CMake configuration:

cmake -S . -B build-199-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-199-patch --parallel 1
ctest --test-dir build-199-patch --output-on-failure

Result:

100% tests passed, 0 tests failed out of 5

Notes

I do not have access to an ARM strict-alignment target for runtime validation, so this follows the issue's proposed fix while keeping the .vnc substitution path unchanged.

Closes #199.