Fix an off-by-1 error for copying overlapping bitmaps in r346416. For

nonzero height, the first line in the original order was not copied, and
for zero height, garbage lines before the first were copied until a crash
occurred.

lib/libvgl/bitmap.c

@@ -207,7 +207,7 @@ __VGLBitmapCopy(VGLBitmap *src, int srcx, int srcy,
   yextra = 0;
   ystep = 1;
   if (src->Bitmap == dst->Bitmap && srcy < dsty) {
-    yend = srcy;
+    yend = srcy - 1;
     yextra = hight - 1;
     ystep = -1;
   }