Converting to RGB might involve a precision increase

Need to do more than a simple shift to get the appropriate
new value. This isn't quite as exact as a proper multiplication
and division, but the error is so small it's not worth the
extra cycles.
diff --git a/common/rfb/PixelFormat.h b/common/rfb/PixelFormat.h
index aef309c..dd41474 100644
--- a/common/rfb/PixelFormat.h
+++ b/common/rfb/PixelFormat.h
@@ -89,9 +89,9 @@
     int blueShift;
 
   protected:
-    int redConvShift;
-    int greenConvShift;
-    int blueConvShift;
+    /* Pre-computed values to keep algorithms simple */
+    int redBits, greenBits, blueBits;
+    int maxBits, minBits;
     bool endianMismatch;
   };
 }