wrbmp.c: Use IsExtRGB() macro where appropriate

(to improve readability)
diff --git a/wrbmp.c b/wrbmp.c
index 38a64e8..4bf8142 100644
--- a/wrbmp.c
+++ b/wrbmp.c
@@ -5,7 +5,7 @@
  * Copyright (C) 1994-1996, Thomas G. Lane.
  * libjpeg-turbo Modifications:
  * Copyright (C) 2013, Linaro Limited.
- * Copyright (C) 2014-2015, 2017, D. R. Commander.
+ * Copyright (C) 2014-2015, 2017, 2019, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -303,9 +303,7 @@
   int bits_per_pixel, cmap_entries;
 
   /* Compute colormap size and total file size */
-  if (cinfo->out_color_space == JCS_RGB ||
-      (cinfo->out_color_space >= JCS_EXT_RGB &&
-       cinfo->out_color_space <= JCS_EXT_ARGB)) {
+  if (IsExtRGB(cinfo->out_color_space)) {
     if (cinfo->quantize_colors) {
       /* Colormapped RGB */
       bits_per_pixel = 8;
@@ -499,9 +497,7 @@
 
   if (cinfo->out_color_space == JCS_GRAYSCALE) {
     dest->pub.put_pixel_rows = put_gray_rows;
-  } else if (cinfo->out_color_space == JCS_RGB ||
-             (cinfo->out_color_space >= JCS_EXT_RGB &&
-              cinfo->out_color_space <= JCS_EXT_ARGB)) {
+  } else if (IsExtRGB(cinfo->out_color_space)) {
     if (cinfo->quantize_colors)
       dest->pub.put_pixel_rows = put_gray_rows;
     else