Option: shader

Summary: OpenGL pixel shader
Default: 0
Example: curvature
Since: 1.3.0

FIXME: Consider splitting this option into shader_name (for bundled shader)
and shader_path (for external shader)?

The option shader be either the name of a bundled shader (without .shader
extension), or path to a shader in XML Shader Format 1.1, including the .shader
file extension.

The bundled shaders are:

* atari-color-fix
* crt-guest
* crt-hyllian
* crt-lottes
* crt
* curvature
* edge-detection
* hard-light-bloom-gamma
* hard-light-bloom
* hard-light
* heavybloom
* horizontal-blur
* hq2x
* lanczos-6tap
* scale2x
* scale4xhq
* scalefx
* scanline-3x
* scanline-4x
* scanlines-classic
* scanlines-nonlinear-blur
* scanlines-nonlinear
* screenlines-nonlinear-blur-hsl-thick
* screenlines-nonlinear-blur-hsl
* screenlines-nonlinear-blur-thick
* screenlines-nonlinear-blur
* screenlines-nonlinear
* sharp-bilinear-simple
* sharp-bilinear
* simplebloom
* super-xbr-3p
* test-gamma-screenlines
* test-gamma
* test
* testbloom-2
* testbloom
* xbrz4x
* xbrz6x