Default:
Example: curvature
Since: 1.3.0

The option shader be either the name of a bundled shader, or path to a
shader in XML Shader Format 1.1. The bundled shaders are:

* crt
* curvature
* edge-detection
* heavybloom
* hq2x
* lanczos-6tap
* scale2x
* scale4xhq
* scanline-3x
* scanline-4x
* simplebloom
