image size defined with Session.set_view_area from retina and non-retina display

[kswang1029]

To avoid confusion and support better compatibility with retina (deviceRatio=2 for mac) and non-retina display, when we try
session.set_view_area(800, 800), for example, we should keep the size of the output PNG as 800x800. Currently, if a Retina display is detected, we will see the resulting PNG has a size of 1600x1600.

example script

from carta.session import Session
from carta.browser import Chrome

url_without_token = 'http://localhost:3003'
image_file_0 = 'HD163296_CO_2_1_subimage.fits'

session = Session.create(Chrome(), url_without_token, debug_no_auth=True)
session.set_view_area(800, 800)
img0 = session.open_image(image_file_0)
img0.set_channel(88)
img0.set_colormap('tab10')
session.save_rendered_view('create_session.png')