They are not transitionable:
https://drafts.csswg.org/css-transitions-1/#transitionable
There are some new failures in background-image-interpolation.html,
but I think the test is wrong, because it expects background-image
to be transitionable, even though the spec defines it with a discrete
animation type.
PlacementAmongFloats would stop iterating when current_bands would be
empty, even if next_band wasn't at infinity.
Then the BFC root or replaced block was placed after all the floats,
even if it could fit next to some of them.
This patch moves the next_band into current_bands so that the loop
keeps considering bands.
PlacementAmongFloats was assuming that there would always be a FloatBand
at a position smaller than or equal to the given ceiling, but this was
not the case for negative ceilings.
The reason is that the FloatContext initialized the FloatBandTree with
a band at 0, and another at +∞.
This patch changes the initial bands to −∞ and +∞. This seems more
consistent and matches the expectation of PlacementAmongFloats.
