[Feature] Expose PSPrite's stencilcolor or add A_OverlayShade/SetShade method

[jekyllgrim]

GZDoom version

4.12.2

Which game are you running with GZDoom?

None

What Operating System are you using?

Windows 10

If Other OS, please describe

No response

Relevant hardware info

No response

Is your feature request related to a problem? Please describe.

While STYLE_Shaded can be set on a PSprite, there's no field or method that would allow actually modifying its color afterwards, which is very inconvenient.

Describe the solution you'd like

Either a stencilcolor field, or a dedicated SetShade() and/or A_OverlayShade() method.

Describe alternatives you've considered

1. Using color translation (barely an alternative, since translations can't be created on the fly).
2. Using a custom material shader (way more setup, not generic enough).

Add any other context or screenshots about the feature request here.

No response