Replace saveFrameToPng() to fix black halo on exported images

[ashikase]

NOTE: This fixes the following issues:
airbnb/lottie-web#73
airbnb/lottie-web#158

NOTE: This is an improved version of the following pull request:
#1

[ashikase]

As mentioned, this is a cleaned-up and augmented version of the "improve png exporting" pull request submitted by @bigxixi.

Features:

• Uses the render queue to export images for each selected composition.
• Overwrites any existing images from a previous export.
• Handles a bug where "00000" gets added to the end of the exported file's name.
• Backs up the render queue before export, and restores when finished.
• Temporarily disables the "Save when starting render queue" option to prevent export delays.
• Temporarily disables the "Play Sound When Render Finishes" option as it can be annoying when exporting multiple compositions.

Note that this patch was tested with the bodymovin-extension provided in the latest release of lottie-web, version 5.1.10, with Adobe After Effects CC v15.0.1 build 73.

[bodymovin]

@ashikase Thanks for your collaboration!!
I've done some small changes and added more image features on top of it.