There are some issues with immediately tracking readbacks in the global
linked list of pending readbacks:
- The Pass might not get submitted, in which case the readback will be
"dangling" and never complete (or it will erroneously think it's
completed but its buffer will contain garbage data).
- Thread safety issues of modifying a global data structure from a Pass.
Instead, Pass will locally track the readbacks it performs, and only at
submit time will those readbacks get added to the global list.
(There is a little bit of refcounting mistakes now, those will get
cleaned up).
- Rename/reorder some projection matrix functions.
- Make perspective functions flip Y and use 0-1 NDC range.
- Flip winding and font vertices based on handedness.
This stuff is really confusing
Pass stores a small 16-bucket cache of vertices/indices it recently
generated. Draws that have relatively predictable geometry can provide
a hash along with their draw. The Pass will reuse vertices based on the
hash, when possible, and return a NULL vertex pointer to let the draw-er
know they don't need to generate any vertices.
This provides a dramatic speedup when drawing the same shape many times
in a row. The overhead is negligible, with benefits kicking in with
just a small handful of repeated draws (3-5 for cubes, less for more
complex shapes).