heif-enc leaks encoder when using --params

[bradh]

Probably not really an issue when using software encoders, but might be more complicated if there is hardware involved.

$ valgrind --leak-check=full ./examples/heif-enc --params --encoder x265 /usr/share/wallpapers/Dance_of_the_Spirits/contents/images/2560x1600.jpg -o out.heif
==17583== Memcheck, a memory error detector
==17583== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==17583== Using Valgrind-3.21.0 and LibVEX; rerun with -h for copyright info
==17583== Command: ./examples/heif-enc --params --encoder x265 /usr/share/wallpapers/Dance_of_the_Spirits/contents/images/2560x1600.jpg -o out.heif
==17583== 
Parameters for encoder `x265 HEVC encoder (3.5+1-f0c1022b6)`:
  quality, default=50, [0;100]
  lossless, default=false
  preset, default=slow, { ultrafast,superfast,veryfast,faster,fast,medium,slow,slower,veryslow,placebo }
  tune, default=ssim, { psnr,ssim,grain,fastdecode }
  tu-intra-depth, default=2, [1;4]
  complexity, [0;100]
  chroma, default=420, { 420,422,444 }
==17583== 
==17583== HEAP SUMMARY:
==17583==     in use at exit: 464 bytes in 3 blocks
==17583==   total heap usage: 207 allocs, 204 frees, 174,935 bytes allocated
==17583== 
==17583== 464 (16 direct, 448 indirect) bytes in 1 blocks are definitely lost in loss record 3 of 3
==17583==    at 0x4845FA3: operator new(unsigned long) (in /usr/libexec/valgrind/vgpreload_memcheck-amd64-linux.so)
==17583==    by 0x48F0934: heif_context_get_encoder (heif.cc:2133)
==17583==    by 0x111603: main (heif_enc.cc:716)
==17583== 
==17583== LEAK SUMMARY:
==17583==    definitely lost: 16 bytes in 1 blocks
==17583==    indirectly lost: 448 bytes in 2 blocks
==17583==      possibly lost: 0 bytes in 0 blocks
==17583==    still reachable: 0 bytes in 0 blocks
==17583==         suppressed: 0 bytes in 0 blocks
==17583== 
==17583== For lists of detected and suppressed errors, rerun with: -s
==17583== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)