Add fallback for hhea caretSlopeRise and caretSlopeRun to match makeotf

[punchcutter]

From googlefonts/glyphsLib#284, but do it here instead because we want to match what makeotf does, not Glyphs itself.

[anthrotype]

thanks!

[anthrotype]

@punchcutter I made a few small adjustments like using built-in round and supporting the case where only one of the two is defined; I hope you don't mind
https://github.com/googlei18n/ufo2ft/blob/2d42d89b41538acdd89e66faa6c7c475bbea52ff/Lib/ufo2ft/fontInfoData.py#L76-L103

[punchcutter]

@anthrotype I started to add non 1000 options for caretSlopeRise because the hhea spec doesn't care, but makeotf appears to always set it to 1000. It seems odd that anyone would ever manually set only the run or the rise, but this looks good to cover it all. Thanks!

[anthrotype]

it is odd, and I thought about warning and ignore if one of the two was missing.. but then I thought I could just make it work.

[punchcutter]

makeotf has no option to set those manually since they are always calculated from the italic angle, but more control doesn't hurt.