Use bpp from ICO header

[radarhere]

Resolves #4911

In an ICO, if the frame mode is "1", that means that it can't determine bpp, since BIT2MODE has no rawmode of "1".

Pillow/src/PIL/IcoImagePlugin.py

Lines 189 to 195 in 9e6dabf

	# figure out where AND mask image starts
	mode = a[0]
	bpp = 8
	for k, v in BmpImagePlugin.BIT2MODE.items():
	if mode == v[1]:
	bpp = k
	break

Pillow/src/PIL/BmpImagePlugin.py

Lines 39 to 47 in 9e6dabf

	BIT2MODE = {
	# bits => mode, rawmode
	1: ("P", "P;1"),
	4: ("P", "P;4"),
	8: ("P", "P"),
	16: ("RGB", "BGR;15"),
	24: ("RGB", "BGR"),
	32: ("RGB", "BGRX"),
	}

Instead, allow the file to report its bpp. Test image created using GIMP.