fix shader marking issue

src/mol-gl/shader/chunks/apply-marker-color.glsl

 // only mark elements with an alpha above the picking threshold
 if (uAlpha >= uPickingAlphaThreshold) {
-    float marker = vMarker * 255.0;
+    float marker = floor(vMarker * 255.0 + 0.5); // rounding required to work on some cards on win
     if (marker > 0.1) {
         if (intMod(marker, 2.0) > 0.1) {
             gl_FragColor.rgb = mix(uHighlightColor, gl_FragColor.rgb, 0.3);