commit 462e5f71933d2cb48cd0a8f042f6428a6b22807c
author L. David Baron <dbaron@chromium.org> Mon Jun 07 22:28:21 2021
committer Chromium LUCI CQ <chromium-scoped@luci-project-accounts.iam.gserviceaccount.com> Mon Jun 07 22:28:21 2021
tree e54146b07613d71268951ba2bfacdd630caf03c5
parent 0d601e69ad7756491dba04af4e83848f0779c2ed

Keep 3D points in a quad coplanar when clamping them to reasonable values.

This is necessary because sending quads that aren't coplanar (or,
really, within rounding error of being coplanar) to the plane splitting
code (DrawPolygon::SplitPolygon) confuses the plane splitting code (for
good reason) and leads to incorrect splits that can leave some pieces
undrawn.

There are two major pieces to this change:

 * First, it changes the code that finds the appropriate transition
   point on an edge that passes between positive and negative w (as part
   of eliminating the parts of the quad with negative w) to choose a
   point that is appropriately-clamped rather than nearly-infinite.

 * Second, it adds new clamping code to clamp any remaining out-of-range
   values (or the results of the first part if rounding puts them
   slightly out of range) to clamp by moving them inwards within the
   plane.  Before doing this, if needed, it may also (if none of the
   plane is close enough to the origin at x=0 and y=0) to move z
   coordinates on all of the points in the plane a bit closer to z=0.

Fixed: 1202695
Change-Id: I82b9510710439a81135559ee7d686480cd1f30ff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2877604
Reviewed-by: Robert Flack <flackr@chromium.org>
Reviewed-by: Kevin Ellis <kevers@chromium.org>
Commit-Queue: David Baron <dbaron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#889969}