Support negative height in BMP files

The BMP file format standard allows for the bitmap height to be defined as a negative number. When negative, it means that instead of the origin being at the lower left it is instead at the upper left when the height is negative. This file change supports those images where the origin is in the upper left.

Author: cogliano

adafruit_imageload/bmp/indexed.py

@@ -59,7 +59,13 @@ def load(file, width, height, data_start, colors, color_depth, *, bitmap=None, p
         while colors > 2 ** minimum_color_depth:
             minimum_color_depth *= 2
 
-        bitmap = bitmap(width, height, colors)
+        #convert unsigned int to signed int when height is negative
+        if height > 0x7fffffff:
+    		height = height - 4294967296
+        theight = height
+        if theight < 0:
+            theight = 0 - theight
+        bitmap = bitmap(width, theight, colors)
         file.seek(data_start)
         line_size = width // (8 // color_depth)
         if width % (8 // color_depth) != 0:
@@ -69,8 +75,15 @@ def load(file, width, height, data_start, colors, color_depth, *, bitmap=None, p
 
         chunk = bytearray(line_size)
         mask = (1 << minimum_color_depth) - 1
-
-        for y in range(height - 1, -1, -1):
+        if height > 0:
+            range1 = height - 1
+            range2 = -1
+            range3 = -1
+        else:
+            range1 = 0
+            range2 = abs(height)
+            range3 = 1
+        for y in range(range1, range2, range3):
             file.readinto(chunk)
             pixels_per_byte = 8 // color_depth
             offset = y * width