negative height check doesn't work without longint

[dhalbert]

@FoamyGuy pointed out in discord that

Adafruit_CircuitPython_ImageLoad/adafruit_imageload/bmp/indexed.py

Lines 75 to 76 in 0c1a28a

	if height > 0x7FFFFFFF:
	height = height - 4294967296

causes an error on import if longints aren't enabled (e.g. on pewpew_m4).

@cogliano pinging you as you added the PR that supports negative heights. Perhaps something can be done earlier in the call chain

[kevinjwalters]

Any thoughts on CLUE OnDiskBitmap returning strange values. He's got a negative height bmp there.