handle negative height bitmaps and ensure that we read in bmp dimensions as signed ints

lib/fastimage.rb

@@ -413,8 +413,17 @@ class FastImage
   end
 
   def parse_size_for_bmp
-    d = get_chars(29)[14..28]
-    d.unpack("C")[0] == 40 ? d[4..-1].unpack('LL') : d[4..8].unpack('SS')
+    d = get_chars(32)[14..28]
+    header = d.unpack("C")[0]
+    
+    result = if header == 40
+               d[4..-1].unpack('l<l<')
+             else
+               d[4..8].unpack('SS')
+             end
+
+    # ImageHeight is expressed in pixels. The absolute value is necessary because ImageHeight can be negative
+    [result.first, result.last.abs]
   end
 
   def get_exif_byte_order

test/test.rb

@@ -12,6 +12,7 @@ FixturePath = File.join(PathHere, "fixtures")
 
 GoodFixtures = {
   "test.bmp"=>[:bmp, [40, 27]],
+  "test2.bmp"=>[:bmp, [1920, 1080]],
   "test.gif"=>[:gif, [17, 32]],
   "test.jpg"=>[:jpeg, [882, 470]],
   "test.png"=>[:png, [30, 20]],