Icchen/81 high resolution export

[I-Chenn]

@confluence
This draft PR is to resolve #81.

An argument image_ratio and several steps are added into save_rendered_view to output png-image with different image ratio:
Setting setIsExportingImage to True enables setting output image ratio by calling setImageRatio. After writing the data to the file, we set setIsExportingImage to False and reset the image ratio to 1 with resetImageRatio.

The new-configured function does work for a new opened session. With the script like:

session.save_rendered_view(file_name="/home/icchen/Pictures/test/test_4.png", image_ratio=4)

I am able to generate a reolution = 400% png image like:

Nevertheless, there's something really odd: after a period of time (say 20 - 30 mins) without touching anything, the exact same execution will then generate something like:

The image is still with the same 400% resolution in size, but the output content seems to lock to 100% resolution.

I still have no idea what's the reason that causes the phenomena, but I will discuss with @YuHsuan-Hwang and see if there's any mechanism at frontend will causes that.

[confluence]

The image ratio parameter should be added to all three functions for retrieving data -- rendered_view_url, rendered_view_data as well as save_rendered_view. The user could access any of these stages directly. setImageRatio, resetImageRatio and setIsExportingImage should be called from the innermost function (rendered_view_url).

[I-Chenn]

@confluence I made some modifications according to your suggestions, the issue mentioned above (lock to 100% resolution issue after 20 - 30 mins) is still appearing. I firstly thought it probably due to: we reset the image_ratio to 1 not until the data is written into the png file (and that's the reason I want to call resetImageRatio in save_rendered_view after the writing file line). Nevertheless, it seems that the issue still appears if I do so. Will keep investigating for the cause of the issue.

[I-Chenn]

@confluence, another issue is that we need to restrict the image_ratio. Do you think if it's proper to set 5 as the upper limit for image_ratio?