stepmania: fix aarch64-linux build by using libpng from nixpkgs

Currently, the aarch64-linux build of Stepmania fails with libpng linkage errors:

> ld: ../extern/libpng.a(pngrtran.c.o): in function `png_do_read_transformations':
> pngrtran.c:(.text+0x4118): undefined reference to `png_riffle_palette_neon'
> ld: pngrtran.c:(.text+0x5188): undefined reference to `png_do_expand_palette_rgba8_neon'
> ld: pngrtran.c:(.text+0x6268): undefined reference to `png_do_expand_palette_rgb8_neon'
> ld: ../extern/libpng.a(pngrutil.c.o): in function `png_read_filter_row':
> pngrutil.c:(.text+0x6734): undefined reference to `png_init_filter_functions_neon'
> collect2: error: ld returned 1 exit status

Stepmania vendors many dependencies in source form in their code repository; it appears that the libpng
build configured there seems to be either misconfigured or isn't building correctly under NixOS. I played
around with a couple of NEON-related compile flags but couldn't get anything to stick.

A workaround that works seems to be building against the Nixpkgs libpng instead. It builds, launch, and (so far)
plays correctly on my NixOS-based M1 Air and my PC.

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