Only re-render the selection overlay once per batch of requests

Also, decrease the maximum latency between a render request and when the update is performed to be 0ms. The previous value of 200ms was quite noticeable when indenting or unindenting paragraphs.

# Performance
@ 1000pages.odt - Chrome

|    | master 0b4f6a6 (ms) | pr 329 (ms) |
|----------------------------------|-------|-----|
|Select the entire document|1848 |721|
|Bold the current selection  |6992 |6014|