Rework internals of QgsTextRenderer horizontal text rendering,
so that:

1. It's easier to read, cleaning up years of accumulated cruft
2. Split out bits into smaller functions
3. Reduce the amount of duplicate code for text layout, so that
there's only one function responsible for horizontal text layout
instead of multiple
4. Add shortcut optimisations for rendering text + buffer/shadow
at the same time, instead of always drawing these completely
independantly of each other and incurring the cost of text path
calculation multiple times for the same bit of text. In a simple
benchtest this reduces the time required for rendering many
text fragments with buffers from 40 seconds to 27 seconds.

There's still more we could do here, but it's a start...

Co-authored-by: Mathieu Pellerin <[email protected]>