Skybox image: decode PNG ourselves to avoid symbol interposition

client.so, engine.so and vguimatsurface.so all export their own
png_create_read_struct. Loading the system libpng16 made its internal calls
bind to the game's older libpng, whose png_struct layout differs, so
png_safe_warning read a NULL error_ptr and crashed.

Replace libpng with a small PNG reader: zlib's uncompress (dlopen'ed) plus the
five PNG row filters, supporting 8-bit non-interlaced RGB/RGBA/gray/palette
images with tRNS alpha for palettes. Output verified byte-identical to libpng
for the skybox.png in use.

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