Support BoxFit

[tinyjin]

When the canvas size is bigger than animation has, image is resized like Boxfit.fill. This issue causes different size from lottie-flutter.

Step 1. Must ensure the default rendering result is Boxfit.none.

Step 2. Should support 7 options on BoxFit enum: https://api.flutter.dev/flutter/painting/BoxFit.html

• fill
• contain
• cover
• fitWidth
• fitHeight
• none
• scaleDown

The parameter usage will be:

Lottie.asset(
    'assets/path/to/lottie.json',
    width: 300,
    height: 300,
    fit: BoxFit.cover
)