Fix issues w/ partial img decompr + buf img mode

Fixes #611

ChangeLog.md

@@ -23,6 +23,15 @@ iMCU (8 * the vertical sampling factor) using buffered-image mode with
 interblock smoothing enabled.  This was a regression introduced by
 2.1 beta1[6(b)].
 
+5. Fixed two issues that prevented partial image decompression from working
+properly with buffered-image mode:
+
+     - Attempting to call `jpeg_crop_scanline()` after
+`jpeg_start_decompress()` but before `jpeg_start_output()` resulted in an error
+("Improper call to JPEG library in state 207".)
+     - Attempting to use `jpeg_skip_scanlines()` resulted in an error ("Bogus
+virtual array access") under certain circumstances.
+
 
 2.1.3
 =====

jdapistd.c

@@ -163,7 +163,8 @@ jpeg_crop_scanline(j_decompress_ptr cinfo, JDIMENSION *xoffset,
   my_master_ptr master = (my_master_ptr)cinfo->master;
 #endif
 
-  if (cinfo->global_state != DSTATE_SCANNING || cinfo->output_scanline != 0)
+  if ((cinfo->global_state != DSTATE_SCANNING &&
+       cinfo->global_state != DSTATE_BUFIMAGE) || cinfo->output_scanline != 0)
     ERREXIT1(cinfo, JERR_BAD_STATE, cinfo->global_state);
 
   if (!xoffset || !width)
@@ -525,7 +526,7 @@ jpeg_skip_scanlines(j_decompress_ptr cinfo, JDIMENSION num_lines)
    * all of the entropy decoding occurs in jpeg_start_decompress(), assuming
    * that the input data source is non-suspending.  This makes skipping easy.
    */
-  if (cinfo->inputctl->has_multiple_scans) {
+  if (cinfo->inputctl->has_multiple_scans || cinfo->buffered_image) {
     if (cinfo->upsample->need_context_rows) {
       cinfo->output_scanline += lines_to_skip;
       cinfo->output_iMCU_row += lines_to_skip / lines_per_iMCU_row;