Include leading margin in inline block size

According to the documentation for Fragment::position, the inline axis
should include margin size, so we include it for blocks. Also fix
place_float which assumed that it was not included and
assign_inline_sizes which overrode the size set in
set_inline_size_constraint_solutions.

Typically this issue was hidden by large tile sizes, but fitted tiles
makes it more common.
auto merge of #3618 : mrobinson/servo/layer-sizing, r=pcwalton

Extra size from margins should be included in block size, so that
layers are large enough to include the entire block. This is typically
hidden by large tile sizes (512x512), but fitted tiles makes the issue
a lot more common.