stbi: update thirdparty/stb_image to latest commit ae721c5

[ttytm]

Update stbi to latest commit https://github.com/nothings/stb/tree/ae721c50eaf761660b4f90cc590453cdb0c2acd0.

Vab currently misses a cflag for andorid compilation to work.
Instead of making it a requirement to add it manually when compiling or running CI, this PR to adds it directly into vab: vlang/vab#278.

Tested on a fork with a temporary change to use the vab fork of the linked PR above

# vab_ci.yml
# ...
# Line 28
      - name: Install vab
        run: |
-         .github/workflows/retry.sh v install vab
+         .github/workflows/retry.sh v install https://github.com/ttytm/vab@update-stbi-cflags

Result: https://github.com/ttytm/v/actions/runs/8469154352

So the only requirement should be merging vlang/vab#278 then re-running the full VAB workflow should be green.