add a sniffer for TTF

since TTF files can be caught by the almost unfixably overgenerous TGA sniffer

see https://github.com/libvips/php-vips/issues/127

ChangeLog

@@ -5,6 +5,7 @@
 - fix a crash with 0 length vectors 
 - change default frame delay for GIFs from 1s to 0.1s
 - remove stray trailing comma from iiif3 dirnames [whalehub]
+- fix TTF load [chregu]
 
 21/11/21 started 8.12.1
 - fix insert [chregu]

libvips/foreign/magick.c

@@ -82,6 +82,14 @@ magick_sniff( const unsigned char *bytes, size_t length )
 		bytes[3] == 0 )
 		return( "ICO" );
 
+	if( length >= 5 &&
+		bytes[0] == 0 && 
+		bytes[1] == 1 &&
+		bytes[2] == 0 &&
+		bytes[3] == 0 &&
+		bytes[4] == 0 )
+		return( "TTF" );
+
 	if( length >= 18 &&
 		(bytes[1] == 0 || 
 		 bytes[1] == 1) &&