added a wrapper for alpha_over that falls back on PIL paste() if needed

composite.py

@@ -0,0 +1,51 @@
+#    This file is part of the Minecraft Overviewer.
+#
+#    Minecraft Overviewer is free software: you can redistribute it and/or
+#    modify it under the terms of the GNU General Public License as published
+#    by the Free Software Foundation, either version 3 of the License, or (at
+#    your option) any later version.
+#
+#    Minecraft Overviewer is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
+#    Public License for more details.
+#
+#    You should have received a copy of the GNU General Public License along
+#    with the Overviewer.  If not, see <http://www.gnu.org/licenses/>.
+
+import logging
+
+from PIL import Image
+
+"""
+This module has an alpha-over function that is used throughout
+Overviewer. It defaults to the PIL paste function when the custom
+alpha-over extension cannot be found.
+"""
+
+extension_alpha_over = None
+try:
+    from _composite import alpha_over as _extension_alpha_over
+    extension_alpha_over = _extension_alpha_over
+except ImportError:
+    logging.warning("alpha_over extension not found; using default PIL paste()")
+
+def alpha_over(dest, src, pos_or_rect=(0, 0), mask=None):
+    """Composite src over dest, using mask as the alpha channel (if
+    given), otherwise using src's alpha channel. pos_or_rect can
+    either be a position or a rectangle, specifying where on dest to
+    put src. Falls back to dest.paste() if the alpha_over extension
+    can't be found."""
+    if mask == None:
+        mask = src
+    
+    global extension_alpha_over
+    if extension_alpha_over != None:
+        # extension ALWAYS expects rects, so convert if needed
+        if len(pos_or_rect) == 2:
+            pos_or_rect = (pos_or_rect[0], pos_or_rect[1], src.size[0], src.size[1])
+        extension_alpha_over(dest, src, pos_or_rect, mask)
+    else:
+        # fallback
+        dest.paste(src, pos_or_rect, mask)
+