Small refactoring of paeth filter logic

[fintelia]

Better naming for each of the paeth_filter methods:

• filter_paeth: Straightforward implementation as described in the PNG spec
• filter_paeth_stbi: Version from STB Image
• filter_paeth_stbi_i16: Same as above, but uses i16's as input/output
• filter_paeth_fpnge: Version from FPNGE

Also fixes an edge case where enabling the "unstable" feature on non-x86_64 architectures would skip filtering, and adjusted the code structure to make similar issues less likely in the future.