Removed DPI rounding from BMP, JPEG and PNG loading

[radarhere]

Helps #4129

BMP and PNG don't save our dpi value untransformed. Instead, BMP multiplies it by 39.3701 and PNG divides it by 0.0254, and then saves it as an integer. Because this limits the precision, that makes it unlikely that dpi value used to save an image will be the same dpi that is loaded afterwards - this means that this PR abandons the idea that you should be able to roundtrip DPI values. So

from PIL import Image
im = Image.new("RGB", (100, 100))
im.save("out.png", dpi=(50, 50))

im = Image.open("out.png")
print(im.info["dpi"])  # (50.0126, 50.0126)

After that, it removes rounding from JPEG loading as well. This is more of a cleanup than a change though, because as it is an integer when saving, I don't think it is possible to have a JPEG with a float dpi without hexediting the image.