Bugfix/background property targeting

[chanatorns]

Hello again, I found that we cannot pick color from css background image. After checking carefully I understand that the 'elementToCanvas' was created to support picking color from css's background image, but it is still have some cases that are not supported, for example...

• If background image is scaled up or down it will unable to pick correct pixel as image in canvas is use its original size.
• If background image is repeated it will unable to pick color from the reapeated image because in canvas it show only single image.

I found that html2canvas is able to support picking color form css's background image nicely and able to support above cases. Only concern I previously had is performance, anyway after testing recently I didn't found any performance issue. So I think we can let html2canvas handle this and we can remove 'elementToCanvas'. This is what's changed in this PR.

1. Fix to support picking color from css's background image.
2. Add more example to show case about css background image.
3. Update enzyme adapter to be the same version with current react's version with v17

[jodhman]

Hello @chanatorns! Sounds good, will make another release within the days.

[chanatorns]

@jodhman Thankyou! looking forward to it.

[jodhman]

Version 5.5.1 released on npm! :rocket: