Depends on #904.
Add BackgroundColor, BorderColor, BorderWidth, CornerRadius and Padding
properties.
Modularize button painting code.
Paint border after background to get slightly better border appearance.
Update screenshots.
Add screenshot test using properties.
Remove nv-flip dependency and use pixel-by-pixel comparison instead.
Reduce error tolerance in tests.
Update a few outdated screenshots.
Create `assert_failing_render_snapshot` macro to make sure our diffing
infrastructure catches regressions.
Fixes#893.
Olivier FAURE