Fix: Overlay: Facing overlay is not transparent with zero alpha

The recent changes removed the blending when rendering
to the framebuffer. The blending was doing the
premultiplication.

Fix #122470

source/blender/draw/engines/overlay/shaders/overlay_facing_frag.glsl

@@ -5,4 +5,6 @@
 void main()
 {
   fragColor = gl_FrontFacing ? colorFaceFront : colorFaceBack;
+  /* Pre-multiply the output as we do not do any blending in the framebuffer. */
+  fragColor.rgb *= fragColor.a;
 }