Improve jpeg dataset opening

[Even Rouault]

To keep track on the "gdal fails on vrt (virtual raster) file" discussion on gdal dev list :

About the speed issue, in fact, jpeglib.h is very clear :

  /* Basic description of image --- filled in by jpeg_read_header(). */
  /* Application may inspect these values to decide how to process image. */

  JDIMENSION image_width;       /* nominal image width (from SOF marker) */
  JDIMENSION image_height;      /* nominal image height */
  int num_components;           /* # of color components in JPEG image */
  J_COLOR_SPACE jpeg_color_space; /* colorspace of JPEG image */


And we are lucky there, since these are just the values we need for 
JPGDataset::Open !

So, we can defer the call to jpeg_start_decompress to the first call to 
JPGDataset::LoadScanline.
See the following patch :

Index: frmts/jpeg/jpgdataset.cpp
===================================================================
--- frmts/jpeg/jpgdataset.cpp   (révision 12079)
+++ frmts/jpeg/jpgdataset.cpp   (copie de travail)
@@ -87,6 +87,7 @@
     int           bSwabflag;
     int    nTiffDirStart;
     int    nTIFFHEADER;
+    int    bHasDoneJpegStartDecompress;

     CPLErr LoadScanline(int);
     void   Restart();
@@ -807,6 +808,7 @@
     adfGeoTransform[3] = 0.0;
     adfGeoTransform[4] = 0.0;
     adfGeoTransform[5] = 1.0;
+    bHasDoneJpegStartDecompress = FALSE;

     poMaskBand = NULL;
     pabyBitMask = NULL;
@@ -849,6 +851,12 @@
     if( nLoadedScanline == iLine )
         return CE_None;

+    if (!bHasDoneJpegStartDecompress)
+    {
+        jpeg_start_decompress( &sDInfo );
+        bHasDoneJpegStartDecompress = TRUE;
+    }
+
     if( pabyScanline == NULL )
         pabyScanline = (GByte *)
             CPLMalloc(GetRasterCount() * GetRasterXSize() * 2);
@@ -1391,8 +1399,6 @@
         return NULL;
     }

-    jpeg_start_decompress( &(poDS->sDInfo) );
-
 /* -------------------------------------------------------------------- */
 /*      Capture some information from the file that is of interest.     */
 /* -------------------------------------------------------------------- */


It seems to work. gdalinfo is much faster on big jpeg files and openev still 
works.

[Even Rouault]

Commited in trunk in r12080