Better inversible transformations

[kvark]

Hopefully addresses #2911 (how can I verify?)
r? @nical

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This change is 

[nical]

Looks good to me (with the extra commits removed). To test it you can add this patch on top of this revision https://treeherder.mozilla.org/#/jobs?repo=try&revision=04c6bc71074245fa9d8438a78922e71fbd36dff7&selectedJob=188748580 and run the crashtests locally ./mach crashtest.

[nical]

@bors-servo r+

[bors-servo]

📌 Commit 2355996 has been approved by nical

[staktrace]

I started a try push that includes #2912 and this PR: https://treeherder.mozilla.org/#/jobs?repo=try&revision=3623dff5d67e5c5a5c77fb5f7a6ed55f7f69cd06

[bors-servo]

⌛ Testing commit 2355996 with merge 412a0b5...

[staktrace]

This patch appears to be insufficient, we're still getting panics (see try push above). It's hitting the all the points are supposed to be transformable after near plane clipping panic path.

[kvark]

@staktrace this makes sense. I'm still working (closest WR bug is #2272) on that part of code (just switching on and off, mostly off, occasionally), and I know it's not yet entirely correct, so unwrapping does no good there (until it's fixed). How about a temporary workaround in 14ebffb ?

[staktrace]

Started a new try push with that: https://treeherder.mozilla.org/#/jobs?repo=try&revision=78d838b9b1f8b0d9e014fe5f2ee05f53135aa2e9

[staktrace]

Above try push still had failures, this time from this line per my local repro on layout/reftests/transform-3d/1035611-1.html

[kvark]

@staktrace thanks for tracing this crash down! I updated the second commit here to somewhat work around it.

Note: the workaround is super unsound and in normal circumstances shouldn't be merged. However, the old code was essentially producing undefined values (behind the near plane) by that transformation, so defaulting to concrete ones, while still wrong, is a step forward.

[staktrace]

I did a local build with that patch (try still going here) and it crashes still. Same test as last time, but a different spot:

#10 0x00007fffe95c50bc in _$LT$core..option..Option$LT$T$GT$$GT$::unwrap::h85c542007de8ef77 (self=...) at /checkout/src/libcore/macros.rs:20
#11 0x00007fffe95cab79 in webrender::batch::make_polygon::h377795bdcde61f94 (rect=..., transform=<optimized out>, anchor=<optimized out>) at gfx/webrender/src/batch.rs:1717
#12 0x00007fffe95c8c34 in webrender::batch::AlphaBatchBuilder::add_prim_to_batch::h74e57828c73cc409 (self=0x7fffc15fa300, transform_id=..., prim_index=..., ctx=<optimized out>, gpu_cache=0x7fffc15fc9b0, render_tasks=0x7fffc15fa660, task_id=..., task_address=..., deferred_resolves=0x7fffc15fa648, splitter=0x7fffc15f8170, content_origin=..., prim_headers=0x7fffc15fa748)
    at gfx/webrender/src/batch.rs:676
#13 0x00007fffe95c88cb in webrender::batch::AlphaBatchBuilder::add_run_to_batch::hfc4831300e0e33d3 (self=0x7fffc15fa300, run=<optimized out>, transform_id=..., ctx=0x7fffc15fa5d0, gpu_cache=<optimized out>, render_tasks=<optimized out>, task_id=..., task_address=..., deferred_resolves=<optimized out>, splitter=<optimized out>, content_origin=..., prim_headers=<optimized out>)
    at gfx/webrender/src/batch.rs:559

webrender/webrender/src/batch.rs

Line 1717 in 2cb6825

Polygon::from_transformed_rect(rect.cast(), mat, anchor).unwrap()

[jrmuizel]

@bors-servo r+

[bors-servo]

📌 Commit a542756 has been approved by jrmuizel

[bors-servo]

⌛ Testing commit a542756 with merge 9f21ee5...

[jrmuizel]

try push:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=716c8d9cf7f08cf51eacc169aff2165e89e5f005

[bors-servo]

☀️ Test successful - status-appveyor, status-taskcluster
Approved by: jrmuizel
Pushing 9f21ee5 to master...

[kvark]

@jrmuizel the errors in this try push appear from #2891:

thread '' panicked at 'Unable to open shader file: "/builds/worker/workspace/build/src/obj-firefox/toolkit/library/x86_64-unknown-linux-gnu/release/build/webrender-fec329b250b1de8f/out/brush_solid.vert": Os { code: 2, kind: NotFound, message: "No such file or directory" }', libcore/result.rs:945:5
11:13:21 INFO - PID 18099 | stack backtrace:

[jrmuizel]

How about this https://treeherder.mozilla.org/#/jobs?repo=try&revision=3e11f519b0e2b4c7fb5475904d7541d3bc37b850