UV: use grey on black for dashed-line display

Dashed line display was changed since 2.7x, using white on black
instead of grey on black.

This made selection difficult to see as white is brighter
than the default selected color.

Match the grey used in 2.7x.

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

@@ -40,7 +40,7 @@ void main()
   }
   else if (lineStyle == OVERLAY_UV_LINE_STYLE_DASH) {
     if (fract(line_distance / dashLength) < 0.5) {
-      inner_color = mix(vec4(1.0), colorEdgeSelect, selectionFac_f);
+      inner_color = mix(vec4(vec3(0.35), 1.0), colorEdgeSelect, selectionFac_f);
     }
   }
   else if (lineStyle == OVERLAY_UV_LINE_STYLE_BLACK) {