## References
* See also #8617
## PR Checklist
* [x] Supports #3075
* [x] I work here.
* [x] Manual test.
## Detailed Description of the Pull Request / Additional comments
### Window Title Generation
Every time the renderer checks the title, it's doing two bad things that
I've fixed:
1. It's assembling the prefix to the full title doing a concatenation.
No one ever gets just the prefix ever after it is set besides the
concat. So instead of storing prefix and the title, I store the
assembled prefix + title and the bare title.
2. A copy must be made because it was returning `std::wstring` instead
of `std::wstring&`. Now it returns the ref.
### Dirty Area Return
Every time the renderer checks the dirty area, which is sometimes
multiple times per pass (regular text printing, again for selection,
etc.), a vector is created off the heap to return the rectangles. The
consumers only ever iterate this data. Now we return a span over a
rectangle or rectangles that the engine must store itself.
1. For some renderers, it's always a constant 1 element. They update
that 1 element when dirty is queried and return it in the span with a
span size of 1.
2. For other renderers with more complex behavior, they're already
holding a cached vector of rectangles. Now it's effectively giving
out the ref to those in the span for iteration.
### Bitmap Runs
The `til::bitmap` used a `std::optional<std::vector<til::rectangle>>`
inside itself to cache its runs and would clear the optional when the
runs became invalidated. Unfortunately doing `.reset()` to clear the
optional will destroy the underlying vector and have it release its
memory. We know it's about to get reallocated again, so we're just going
to make it a `std::pmr::vector` and give it a memory pool.
The alternative solution here was to use a `bool` and
`std::vector<til::rectangle>` and just flag when the vector was invalid,
but that was honestly more code changes and I love excuses to try out
PMR now.
Also, instead of returning the ref to the vector... I'm just returning a
span now. Everyone just iterates it anyway, may as well not share the
implementation detail.
### UTF-8 conversions
When testing with Terminal and looking at the `conhost.exe`'s PTY
renderer, it spends a TON of allocation time on converting all the
UTF-16 stuff inside to UTF-8 before it sends it out the PTY. This was
because `ConvertToA` was allocating a string inside itself and returning
it just to have it freed after printing and looping back around again...
as a PTY does.
The change here is to use `til::u16u8` that accepts a buffer out
parameter so the caller can just hold onto it.
## Validation Steps Performed
- [x] `big.txt` in conhost.exe (GDI renderer)
- [x] `big.txt` in Terminal (DX, PTY renderer)
- [x] Ensure WDDM and BGFX build under Razzle with this change.
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