Fix #130569: Grease Pencil tint modifier should use gradient alpha

Grease pencil tint modifier in gradient mode did not do alpha mix with
the original stroke color, making original color completely black even
when gradient color has alpha. Now fixed.

Pull Request: https://projects.blender.org/blender/blender/pulls/130575

source/blender/modifiers/intern/MOD_grease_pencil_tint.cc

@@ -155,7 +155,8 @@ static ColorGeometry4f apply_gradient_tint(const GreasePencilTintModifierData &t
   const float3 input_rgb = {input_color.r, input_color.g, input_color.b};
   /* GP2 compatibility: ignore vertex group factor and use the plain modifier setting for
    * RGB mixing. */
-  const float3 rgb = math::interpolate(input_rgb, gradient_color.xyz(), tmd.factor);
+  const float3 rgb = math::interpolate(
+      input_rgb, gradient_color.xyz(), tmd.factor * gradient_color.w);
   /* GP2 compatibility: use vertex group factor for alpha. */
   return ColorGeometry4f(rgb[0], rgb[1], rgb[2], factor);
 }