Ensure embedded profile, if any, is always used

Perform sRGB conversion at end of pipe only

withMetadata exports profile, should not convert

Convert one fixture to sRGB to help test

Discovered while investigating #125

src/common.h

@@ -44,6 +44,11 @@ namespace sharp {
   */
   VipsImage* InitImage(ImageType imageType, char const *file, VipsAccess const access);
 
+  /*
+    Does this image have an embedded profile?
+  */
+  bool HasProfile(VipsImage *image);
+
   /*
     Does this image have an alpha channel?
     Uses colour space interpretation with number of channels to guess this.