Make defer strile offset/bytecount loading available at runtime

... and add per-strile offset/bytecount loading capabilities.

Part of this commit makes the behaviour that was previously met when
libtiff was compiled with -DDEFER_STRILE_LOAD available for default builds
when specifying the new 'D' (Deferred) TIFFOpen() flag. In that mode, the [Tile/Strip][ByteCounts/Offsets]
arrays are only loaded when first accessed. This can speed-up the opening
of files stored on the network when just metadata retrieval is needed.
This mode has been used for years by the GDAL library when compiled with
its embeded libtiff copy.

To avoid potential out-of-tree code (typically codecs) that would use
the td_stripbytecount and td_stripoffset array inconditionnaly assuming they
have been loaded, those have been suffixed with _p (for protected). The
use of the new functions mentionned below is then recommended.

Another addition of this commit is the capability of loading only the
values of the offset/bytecount of the strile of interest instead of the
whole array. This is enabled with the new 'O' (Ondemand) flag of TIFFOpen()
(which implies 'D'). That behaviour has also been used by GDAL, which hacked
into the td_stripoffset/td_stripbytecount arrays directly. The new code
added in the _TIFFFetchStrileValue() and _TIFFPartialReadStripArray() internal
functions is mostly a port of what was in GDAL GTiff driver previously.

Related to that, the public TIFFGetStrileOffset[WithErr]() and TIFFGetStrileByteCount[WithErr]()
functions have been added to API. They are of particular interest when
using sparse files (with offset == bytecount == 0) and you want to detect
if a strile is present or not without decompressing the data, or updating
an existing sparse file.
They will also be used to enable a future enhancement where client code can entirely
skip bytecount loading in some situtations

A new test/defer_strile_loading.c test has been added to test the above
capabilities.

1 files changed, 2 insertions, 0 deletions

diff --git a/libtiff/tiffiop.h b/libtiff/tiffiop.h
index 47a553aa..ef63de53 100644
--- a/libtiff/tiffiop.h
+++ b/libtiff/tiffiop.h
@@ -127,6 +127,8 @@ struct tiff {
         #define TIFF_DIRTYSTRIP 0x200000U /* stripoffsets/stripbytecount dirty*/
         #define TIFF_PERSAMPLE  0x400000U /* get/set per sample tags as arrays */
         #define TIFF_BUFFERMMAP 0x800000U /* read buffer (tif_rawdata) points into mmap() memory */
+        #define TIFF_DEFERSTRILELOAD 0x1000000U /* defer strip/tile offset/bytecount array loading. */
+        #define TIFF_LAZYSTRILELOAD  0x2000000U /* lazy/ondemand loading of strip/tile offset/bytecount values. Only used if TIFF_DEFERSTRILELOAD is set and in read-only mode */
 	uint64               tif_diroff;       /* file offset of current directory */
 	uint64               tif_nextdiroff;   /* file offset of following directory */
 	uint64*              tif_dirlist;      /* list of offsets to already seen directories to prevent IFD looping */