is frequently mentioned on Phoronix, mentioned less but used by Cairo (as well as the X.Org Server) for pixel manipulation is the Pixman library. Soeren Sandmann announced a new release candidate of Pixman 0.19.6 this afternoon and it has a few interesting changes worth noting.
New to Pixman are much improved radial and linear gradients, performance improvements, enhanced performance specific to the SSE2 back-end, and a number of bug-fixes and test suite enhancements. The performance improvements should be rather noticeable, especially on ARM platforms.
Pixman 0.19.6 is a development snapshot leading up to the stable Pixman 0.20.0 release. The release announcement and download links for this version of Pixman can be found on xorg-announce