Use g_memdup2() where available and add fallback for older GLib versions

• png: alloc size variable is a png type that's always 32-bit
• vpx: alloc size based on existing allocation
• wavpack: alloc size based on existing allocation

g_memdup() is deprecated since GLib 2.68 and we want to avoid deprecation warnings with recent versions of GLib.

TODO

• flv: ignore, source file unused
• isomp4
• matroska
• wavparse: !994 (merged)
• tests/check
• tests/icles