if seg.name is empty or shorter than four characters, ``strlen(lastFilename) - 4`` underflows (size_t), so the pointer passed to strcmp lands far before the buffer and triggers undefined behavior.
This patch catches too-short segment names and aborts decoding.
* fixed a bug that caused wrong behavior with segment mirroring
(effects must use virtualHeight() / virtualWidth() instead of height() / width())
* added image blur as an option (second slider)
* added very basic error reporting for users
* up to 25% faster, especially with big animated gifs
* made all local variables "static" (don't pollute global namespace)
* drawPixelCallback: cache calculation that do not depend on x/y position
* reduced memory allocations on boards without PSRAM, to avoid crashes
