Implement streamtype=1 option for tables-only JPEG encoding

We already support streamtype=2 to skip producing JPEG tables, but
streamtype=1, which skips everything but the tables, was never implemented.
The streamtype=1 stub code dates to Git pre-history, so it's not
immediately clear why.  Implement the missing support.

jpeg_write_tables() can't resume after a full output buffer (it fails with
JERR_CANT_SUSPEND), so it might seem that Pillow needs to pre-compute the
necessary buffer size.  However, in the normal case of producing an
interchange stream, the tables are written via the same libjpeg codepath
during the first jpeg_write_scanlines() call, and table writes aren't
resumable there either.  Thus, any buffer large enough for the normal case
will also be large enough for a tables-only file.

The streamtype option isn't documented and this commit doesn't change that.
It does add a test though.

Tests/test_file_jpeg.py

@@ -961,6 +961,26 @@ def closure(mode, *args):
             im.load()
             ImageFile.LOAD_TRUNCATED_IMAGES = False
 
+    def test_separate_tables(self):
+        im = hopper()
+        tables = BytesIO()
+        image = BytesIO()
+        im.save(tables, format="JPEG", streamtype=1)
+        im.save(image, format="JPEG", streamtype=2)
+        expectations = (
+            (b"\xff\xc0", False, True),  # SOF0
+            (b"\xff\xc4", True, False),  # DHT
+            (b"\xff\xd8", True, True),  # SOI
+            (b"\xff\xd9", True, True),  # EOI
+            (b"\xff\xda", False, True),  # SOS
+            (b"\xff\xdb", True, False),  # DQT
+        )
+        for marker, in_tables, in_image in expectations:
+            assert (marker in tables.getvalue()) == in_tables
+            assert (marker in image.getvalue()) == in_image
+        im2 = Image.open(BytesIO(tables.getvalue() + image.getvalue()))
+        assert_image_similar(im, im2, 17)
+
     def test_repr_jpeg(self):
         im = hopper()
 

src/libImaging/JpegEncode.c

@@ -218,9 +218,9 @@ ImagingJpegEncode(Imaging im, ImagingCodecState state, UINT8 *buf, int bytes) {
             }
             switch (context->streamtype) {
                 case 1:
-                    /* tables only -- not yet implemented */
-                    state->errcode = IMAGING_CODEC_CONFIG;
-                    return -1;
+                    /* tables only */
+                    jpeg_write_tables(&context->cinfo);
+                    goto cleanup;
                 case 2:
                     /* image only */
                     jpeg_suppress_tables(&context->cinfo, TRUE);
@@ -316,6 +316,7 @@ ImagingJpegEncode(Imaging im, ImagingCodecState state, UINT8 *buf, int bytes) {
             }
             jpeg_finish_compress(&context->cinfo);
 
+cleanup:
             /* Clean up */
             if (context->comment) {
                 free(context->comment);