mac-stats: fix compile error with STDFLOAT_DOUBLE

[skip ci]

pandatool/src/mac-stats/macStatsMonitor.mm

@@ -352,9 +352,9 @@ get_collector_color(int collector_index, bool highlight) {
   }
 
   LRGBColor rgb = PStatMonitor::get_collector_color(collector_index);
-  rgb[0] = encode_sRGB_float(rgb[0]);
-  rgb[1] = encode_sRGB_float(rgb[1]);
-  rgb[2] = encode_sRGB_float(rgb[2]);
+  rgb[0] = encode_sRGB_float((float)rgb[0]);
+  rgb[1] = encode_sRGB_float((float)rgb[1]);
+  rgb[2] = encode_sRGB_float((float)rgb[2]);
 
   PN_stdfloat bright = rgb.dot(LRGBColor(0.2126, 0.7152, 0.0722));
   CGColorRef color = CGColorCreateGenericRGB(rgb[0], rgb[1], rgb[2], 1.0);