Light2D.overlayColour color isn't applied

Light2D.overlayColour field does not behave as expected from the name and it's uses in code.

Several entities in the game seem to have intended tinted colored light overlays, and the name of the field also implies it's a color tint, but that is not what the shader does.

It instead applies the red channel as a darkening factor for the light overlay graphic, dimming it based on how red the color is.

Example: the game tries to apply a purple color to the plasma lamp artifact. The shader receives the whole vec4 R:0.9, G:0.4, B:0.74, A:1.0 as the property _TintColor, but then all values aside _TintColor.r value are discarded, and only the red channel alone is used for the multiplication, resulting in simply shading the light.

Shinebugs have an entire setup to provide them a light color at prefab creation, but due to this issue they are also all grey instead.