fix: mem_ctx should be m->mem_ctx in cute_tiled_free_map_internal

[Phytolizer]

• Fix a typo that causes a compiler error when CUTE_TILED_FREE is defined to use the ctx argument.
• Add some simple code in test_cute_tiled/main.cpp to detect a regression.

[Phytolizer]

Replicate the error like so (copying the regression test without the fix):

[kyle@kyle-desktop-ku test_cute_tiled]$ git show --oneline --no-patch
7c3eb14 (HEAD -> master, origin/master, origin/HEAD) Fix alpha bit shift to use use 0xFFu instead of 0xFF literal (#413)
[kyle@kyle-desktop-ku test_cute_tiled]$ git cherry-pick 2083cf8c73dd4e6aa03991afe2861291bb56a2da
[master d934b59] regression: add "dummy" alloc overrides for test_cute_tiled
 Date: Sun Nov 2 11:09:25 2025 -0600
 1 file changed, 13 insertions(+)
[kyle@kyle-desktop-ku test_cute_tiled]$ make
rm -f tests
g++ -I../ main.cpp -o tests
../cute_tiled.h: In function ‘void cute_tiled_free_map_internal(cute_tiled_map_internal_t*)’:
../cute_tiled.h:2903:79: error: ‘mem_ctx’ was not declared in this scope
 2903 |                         if (desc->animation) CUTE_TILED_FREE(desc->animation, mem_ctx);
      |                                                                               ^~~~~~~
main.cpp:18:51: note: in definition of macro ‘CUTE_TILED_FREE’
   18 | #define CUTE_TILED_FREE(ptr, ctx) dummy_free(ptr, ctx)
      |                                                   ^~~
make: *** [Makefile:2: tests] Error 1