Calling `pixelPositionForScreenPosition` was sometimes throwing an error
indicating that the requested position was not rendered and that, as
such, could not be measured.
This was caused by trying to measure a line that was visible at the
moment of the call while also having a pending autoscroll request that
would cause that line to go off-screen. Due to how the code was
structured, we would mistakenly detect that line as visible, autoscroll
to a different location, re-render a different region of the buffer and
then try to measure the now invisible line.
This commit fixes this issue by restructuring and simplifying the logic
for rendering extra lines in order to measure them. Now, every line for
which a measurement has been requested is stored in a `linesToMeasure`
map. During the first phase of the update process (after honoring
autoscroll requests), we detect which of these lines are currently
visible and if they're not, store them into the
`extraRenderedScreenLines` map, which is then used to render lines that
are invisible but need to be measured.
fc1327eb22