As aforementioned in this news posting and further reiterated in the 0.20.0 release announcement, this major release focuses upon gradient and performance improvements. There's also a great deal of bug-fixes and portability improvements.
When it comes to gradients in Pixman 0.20, radial gradients now follow the PDF specification, large linear gradients are rendered much more accurately, and conical gradients work much better.
In terms of Pixman's 0.20 performance, there is faster image scaling, faster affine transformations, improvements on the ARM NEON platform, and improvements to the SSE2 back-end.
This release is timed quite appropriately for the pending release of X.Org 7.6.