Simplified code

[radarhere]

Suggestions for python-pillow#6747

• I've made a change to WebPImagePlugin, simplifying the code in my opinion.
• I was wondering why your test didn't fail on CentOS 7, until I realised the fill behaviour only became the default in libwebp 0.5.0. So I've noted that in the test.
• If you aren't on board with my stylistic changes to your test code, feel free to disagree.
• I found that our test suite passes if I check if the images are equal, not just similar. Did you find that it didn't pass in some environment, or were you just copying code from another test? I suspect that the lossless argument is what makes this pass.

[ashafaei]

@radarhere Thanks for the pull-request on the pull-request. It took me a while to realize what was going on!
I didn't know about the "equal" function. I checked a few test functions to understand how you do it and tried to make it like them. Everything looks great.