fix missing uniform in direct-volume shader

src/mol-gl/shader/direct-volume.frag.ts

@@ -74,6 +74,7 @@ uniform vec3 uFogColor;
 uniform float uAlpha;
 uniform float uPickingAlphaThreshold;
 uniform bool uTransparentBackground;
+uniform float uXrayEdgeFalloff;
 
 uniform float uInteriorDarkening;
 uniform bool uInteriorColorFlag;