Added --compress option for 'bat pack' command

This compresses all packed Blend files. Other files, as well as already-
compressed Blend files, are left as-is.

blender_asset_tracer/compressor.py

@@ -0,0 +1,77 @@
+"""shutil-like functionality while compressing blendfiles on the fly."""
+
+import gzip
+import logging
+import pathlib
+import shutil
+
+log = logging.getLogger(__name__)
+
+# Arbitrarily chosen block size, in bytes.
+BLOCK_SIZE = 256 * 2 ** 10
+
+
+def move(src: pathlib.Path, dest: pathlib.Path):
+    """Move a file from src to dest, gzip-compressing if not compressed yet.
+
+    Only compresses files ending in .blend; others are moved as-is.
+    """
+    my_log = log.getChild('move')
+    my_log.debug('Moving %s to %s', src, dest)
+
+    if src.suffix.lower() == '.blend':
+        _move_or_copy(src, dest, my_log, source_must_remain=False)
+    else:
+        shutil.move(str(src), str(dest))
+
+
+def copy(src: pathlib.Path, dest: pathlib.Path):
+    """Copy a file from src to dest, gzip-compressing if not compressed yet.
+
+    Only compresses files ending in .blend; others are copied as-is.
+    """
+    my_log = log.getChild('copy')
+    my_log.debug('Copying %s to %s', src, dest)
+
+    if src.suffix.lower() == '.blend':
+        _move_or_copy(src, dest, my_log, source_must_remain=True)
+    else:
+        shutil.copy2(str(src), str(dest))
+
+
+def _move_or_copy(src: pathlib.Path, dest: pathlib.Path,
+                  my_log: logging.Logger,
+                  *,
+                  source_must_remain: bool):
+    """Either move or copy a file, gzip-compressing if not compressed yet.
+
+    :param src: File to copy/move.
+    :param dest: Path to copy/move to.
+    :source_must_remain: True to copy, False to move.
+    :my_log: Logger to use for logging.
+    """
+    srcfile = src.open('rb')
+    try:
+        first_bytes = srcfile.read(2)
+        if first_bytes == b'\x1f\x8b':
+            # Already a gzipped file.
+            srcfile.close()
+            my_log.debug('Source file %s is GZipped already', src)
+            if source_must_remain:
+                shutil.copy2(str(src), str(dest))
+            else:
+                shutil.move(str(src), str(dest))
+            return
+
+        my_log.debug('Compressing %s on the fly while copying to %s', src, dest)
+        with gzip.open(str(dest), mode='wb') as destfile:
+            destfile.write(first_bytes)
+            shutil.copyfileobj(srcfile, destfile, BLOCK_SIZE)
+
+        srcfile.close()
+        if not source_must_remain:
+            my_log.debug('Deleting source file %s', src)
+            src.unlink()
+    finally:
+        if not srcfile.closed:
+            srcfile.close()