Retainer Box's widgets will blend in incorrect space when retainer box uses material shader.

Displaying the UMG content with a material shader will mean that it is drawn to an sRGB aware surface and thus blended in linear space instead of sRGB space (as widgets expect). In sceenshot-1.png the lower text was rendered via retainer box + material.

Steps: 

1. Open attached project
2. PIE

Results: Blending issues that show on the edge of the bottom text sample.