brighten differs from pre v1

Has the parameter value definition changed from Jimp 0.x to 1.x?

# Expected Behavior
Brightness with a value of 0 should not affect image like it did in v0.x
Brightness with a value of 1 should fully lighten an image like it did in v0.x
Brightness with a value of -1 should fully darken an image like it did in v0.x
Brightness with a value of -0.5 should half darken an image like it did in v0.x
Brightness with a value of 0.5 should half lighten an image like it did in v0.x

Using Jimp V0.x (using Node-RED to simplify testing)

# Current Behavior
Brightness with a value of 0 in v1.6 behaves differently - it darkens the image
Brightness with a value of 1 in v1.6 behaves differently - it has little affect on the image
Brightness with a value of -1 correctly fully darkens an image like it did in v0.x
Brightness with a value of -0.5 in v1.6 behaves differently - it FULLY darkens the image
Brightness with a value of 0.5 in v1.6 behaves differently - it half darkens the image


# Failure Information (for bugs)

## Steps to Reproduce

#### Demonstrating -0.5 brightness
1. load image
1. 
2. invert
1. 
3. apply brightness -0.5
1. 

#### Demonstrating -0.0 brightness
1. load image
1. 
2. invert
1. 
3. apply brightness -0.0
1. 

#### Demonstrating 1.0 brightness
1. load image
1. 
2. invert
1. 
3. apply brightness 1.0
1. 

## Context

- Jimp Version: 1.6.0
- Operating System: Win 11
- Node version: 18

## Failure Logs

Start at Jimp's brightness API and reproduce the reported values with Jimp 1.6.0, comparing them with the v0.x behavior described in the issue. Trace how values from -1 through 1 are interpreted and verify that zero, positive, and fractional values match the stated expected behavior when the fix is complete.

Written by the indexing model from the issue text.