fix(frontend): correct appendOffset calculation

[Myriad-Dreamin]

The bug is more easily to observe with #117 not merged, because an empty main file causes single empty page to insert and then the main file recovery causes a bundle of pages to insert.

Bug Reproduce

When appending pages, the svg patches become unreliable on preserving order of elements other that <g/>. To trigger it:

#set page(height: 60pt)
#lorem(30) // append (to ensure that fresh pages appended)
#lorem(30) // append
#lorem(30) // append
#lorem(30) // append
#lorem(30) // append
#lorem(30) // append
#text(fill: red, "123") // never patch color anymore, so it is black because of bug
#text(fill: blue, "123") // never patch color anymore, so it is black because of bug

Possible behavior

Preview will have following behavior because of bug:

glyph resources desync (never happens)

What is interesting is that the first element of <svg/> is never affected, so that preview can always show all glyphs. This is because the document always has at least one page initially.

clip path resources desync (chance to #94)

The resources are stored in the second element of <svg/> will never get updated. This may heavily affect the elements to show correctly across pages, such as cross-page tables.

color resources desync (chance to #104)

The resources stored in the third element of <svg/> will never get updated. One may have change to see that new colors are not effective applied (but old colors are still available).

page out of order (not observable)

It is interesting (but not good) that, the patcher will recover the order of pages after a second patch request coming.

Testcase description

As shown in testcase handleMasterproefThesisAffectedByEmptyPage, with bug fixing, it will become:

[ "insert,1,t0" ] // before
[ "insert,4,t0" ] // after

I also construct another case handleMasterproefThesisAffectedByEmptyPageAntiCase. The patcher works occasionally since all of pages reuse corresponding <g/> elements and no DOM update happens.