Label's shadow outline overlaps when transparent

### Tested versions

Reproducible in 4.2.stable

### System information

v4.2.stable

### Issue description

The Label has a `font_shadow_color` set to a color with some transparency, and a `shadow_outline_size` set. The expected behavior is that the entire shadow, including the outline, would have the color set in font_shadow_color. However, you can see that the shadow outline visibly has a different alpha value than the shadow itself.

### Steps to reproduce

Create a Label node. Set `font_shadow_color` to any color with alpha <1. Set `shadow_outline_size` to some value >0.

### Minimal reproduction project (MRP)

N/A

Start by reproducing the issue with a Label using a translucent font_shadow_color and a shadow_outline_size greater than zero. Trace the Label font-shadow rendering entry point to compare the outline and shadow alpha handling. Done means the entire shadow, including its outline, uses the configured font_shadow_color alpha.

Written by the indexing model from the issue text.