Extract duplicated ASS writing code from libass provider
Add a separate SSA subtitle format and move all of the SSA writing stuff there
Speed up passing subtitles to VSFilter a little

Use temp_file_mapping so that it normally never actually hits the disk
on Windows and use AssSubtitleFormat directly.
Speed up TextFileWriter a bit

Write the text and the newline separately rather than appending the
newline to the text to eliminate a copy of the text.
Push the dropping of non-visible lines into the serialization logic

Makes things way less gross and trivially faster.
Switch to using csri_open_mem
Fix non-pch compilation