Fix T80023 Invisible objects or glitches with object 'in front' + 'X-ray'

Rendering only to the depth buffer seems to need a valid fragment shader
with a color output on some platform.

source/blender/draw/engines/workbench/shaders/workbench_merge_infront_frag.glsl

@@ -8,6 +8,8 @@ out vec4 fragColor;
 void main()
 {
   float depth = texture(depthBuffer, uvcoordsvar.st).r;
+  /* Fix issues with Intel drivers (see T80023). */
+  fragColor = vec4(0.0);
   /* Discard background pixels. */
   if (depth == 1.0) {
     discard;