graphics/libspng: update to 0.7.2

ChangeLog: https://github.com/randy408/libspng/releases/tag/v0.7.2

Fixes

• Re-enabled ARM NEON optimizations for indexed color images
• Fixed encoder options set with spng_set_option() not being respected in some cases
• Fixed spng_get_png_buffer() not setting an error code in some cases
• Fixed encoder heap overflow with specially crafted images (#196)
• Fixed 4GB progressive decode/encode limitation on 32-bit (#198)
• Updated error string for SPNG_EBUFSIZ

New

• Decoder now exposes an estimate of image compression level through SPNG_IMG_COMPRESSION_LEVEL, see Decode options
• Added documentation for all decode and encode options, added notes on encoder configuration

Other

• Added encode fuzzer to ensure robustness of code;
• Fuzzer will run on PR's and continuously on OSS-Fuzz alongside the decode fuzzer
• Updated example code

Reported by: portscout
MFH: 2022q1 (multiple bug fixes)