make test for matlab files more specific

Previously vips just called Mat_Open() to test if a file was a Matlab
save file, but this is rather unreliable. For example, some JPEG files
can crash libmatio, and it can incorrectly think that at least some JP2
files are Matlab save files.

Instead, look for "MATLAB 5.0" at the start of the file. This is really too
specific, the first 116 bytes of a Matlab save file are freeform text,
but in practice all Matlab writers use the first few bytes to record the
file type.

See https://github.com/jcupitt/libvips/issues/385

ChangeLog

@@ -4,6 +4,7 @@
 - bicubic is better on 32-bit int images
 - add pdfload, svgload, gifload for PDF, SVG and GIF rendering 
 - vipsthumbnail knows about pdfload and svgload
+- matload is more specific (thanks bithive)
 
 27/1/16 started 8.2.3
 - fix a crash with SPARC byte-order labq vips images

libvips/foreign/matlab.c

@@ -7,6 +7,8 @@
  * 21/8/14
  * 	- swap width/height
  * 	- set interpretation to rgb16 etc. 
+ * 16/2/16
+ * 	- more specific is_a test 
  */
 
 /*
@@ -319,14 +321,13 @@ vips__mat_load( const char *filename, VipsImage *out )
 int
 vips__mat_ismat( const char *filename )
 {
-	mat_t *mat;
+	unsigned char buf[15];
 
-	vips_error_freeze();
-	mat = Mat_Open( filename, MAT_ACC_RDONLY );
-	Mat_Close( mat );
-	vips_error_thaw();
+	if( vips__get_bytes( filename, buf, 10 ) &&
+		vips_isprefix( "MATLAB 5.0", (char *) buf ) )
+		return( 1 );
 
-	return( mat != NULL );
+	return( 0 );
 }
 
 const char *vips__mat_suffs[] = { ".mat", NULL };

libvips/foreign/matload.c

@@ -122,11 +122,6 @@ vips_foreign_load_mat_class_init( VipsForeignLoadMatClass *class )
 
 	foreign_class->suffs = vips__mat_suffs;
 
-	/* We need to be lower priority than the jpeg loader, since some jpegs
-	 * can make libmatio segv on Mat_Open(). 
-	 */
-	foreign_class->priority = -50;
-
 	load_class->is_a = vips__mat_ismat;
 	load_class->get_flags_filename = 
 		vips_foreign_load_mat_get_flags_filename;