render: fixing markdown li measurement

[gavin-ts]

Summary

Fixes a bug where sometimes the line height wouldn't be used when combining inline elements into a block. Also adds 1px of height for h1 and h2's border bottom.

Details

• fixes nested lists in markdown render slightly mismeasured #647
• adds test
• adds debugging flag to print measured dimensions per element in the tree
• h1 and h2 s were missing 1px for their border bottom
• the li element had an inline text entry for itself (20.125) then a nested ol (24), but it should have used the full line height (24) when combining the text with the ol
• we correctly used the line height when it was only inline elements as children, but we missed the case where it was inline elements followed by a block element
• cleaned up the code so the logic for combining inline elements is clearer

li before

li after

example debugging output

Note: not ideal but still helpful (it goes depth first from body and moves one to the right per depth)
you can inspect the resulting (width, height) and margin top/ bottom values per element (and compare with values in the browser render)

┌--text(51.84375,40.25)
┌-h1(51.84375,50.85) mt:24 mb:16     <- this used to have height 49.85     
┌---text(30.046875,20.125)
┌-----text(30.046875,20.125)
┌----li(62.046875,24) mt:0 mb:0
┌---ol(62.046875,24) mt:0 mb:0
┌--li(94.046875,48) mt:0 mb:0      <- this used to have height 44.125
┌-ul(94.046875,48) mt:0 mb:16     <- so this did too
┌body(94.046875,114.85) mt:24 mb:16

h1 before/after