Function: brighten_image

DOC_MSE_VERSION: since 2.6.0

--Usage--
> brighten_image(input: image, amount: brightening amount)

Brighten or darken an image.

To brighten use an amount between @0@ (no effect) and @1@ (entire image is pure white).

To darken use an amount between @0@ (no effect) and @-1@ (entire image is pure black).

--Parameters--
! Parameter	Type			Description
| @input@	[[type:image]]		Image to brighten/darken.
| @alpha@	[[type:double]]		Brightening factor.