Limit FPX tile size to avoid extending outside image

[radarhere]

FPX doesn't currently have any valid test images, only an image with an invalid number of bands.

Pillow/Tests/test_file_fpx.py

Lines 22 to 25 in e80ddfd

	def test_fpx_invalid_number_of_bands():
	with pytest.raises(OSError, match="Invalid number of bands"):
	with Image.open("Tests/images/input_bw_five_bands.fpx"):
	pass

If I take that image, and hexedit the number of bands to 1 (as seems appropriate for 'input_bw'), then the image opens successfully. Trying to load() it, I then get "ValueError: tile cannot extend outside image".

This PR limits the size of the FPX tiles to avoid extending outside the image, allowing the new image to be loaded.