Incorrect float positioning at page start about vivliostyle.js

[MurakamiShinyu]

In this test case, the problems on page 1 and page 2 are different. There are following two problems:

• Problem 1: The block-start position of the first float's margin edge should be affected by the block-start margin of the first paragraph even though the float element is followed by the paragraph element, because the block-start margin of the paragraph is collapsed with the block-start margin of the body element.

• Problem 2: The block-start margin of floats should not be discarded at unforced page breaks. The CSS spec is not clear about this, but auto-discarding the block-start margin of floats at unforced page breaks causes inconsistent look at page breaks, so we should avoid it.

[MurakamiShinyu]

PR #1297 resolves Problem 2.

• Problem 2: The block-start margin of floats should not be discarded at unforced page breaks. The CSS spec is not clear about this, but auto-discarding the block-start margin of floats at unforced page breaks causes inconsistent look at page breaks, so we should avoid it.

Test: https://gist.githack.com/MurakamiShinyu/ec57c870f6a62348a0b3f279dc4b284b/raw/float-margin-test.html