Clamp the discarded select arm's index in prefix-sum lookups

[lexoliu]

Fixes #1816.

Several prefix-sum lookups have the shape
select(default, sh_array[ix - 1u], ix > 0u). Both arms of select are
evaluated, so lanes with ix == 0 compute sh_array[0u - 1u] before
discarding it. Mali lowers workgroup arrays to ordinary memory, so the
underflowed index becomes a wild address and the driver reports a device loss;
on a Pixel 9 Pro (Immortalis-G715, Android 16) any scene containing a path
lost the device on the first frame. The shaders run unchecked since #1093, so
there is no bounds check to mask it.

This clamps the index in the discarded arm, sh_array[max(ix, 1u) - 1u],
which changes nothing when ix > 0 and keeps the dead access in bounds when
ix == 0. Six sites: one each in tile_alloc and backdrop_dyn, two in
coarse (these four fault on the device), and two in fine which have the
same pattern but didn't fault in my scenes.

Verified on the Pixel 9 Pro with fully unchecked shaders: the scenes that
previously lost the device render correctly. vello_tests passes on Metal, so
the selected values are unchanged. An explicit if would avoid the dual
evaluation altogether; I kept the clamp since it's the smallest diff and it's
what I tested on hardware, but happy to change the shape.

[Copilot]

Pull request overview

Fixes a Vulkan device-loss on Mali GPUs caused by underflowed workgroup-array indexing in the discarded arm of WGSL select expressions used for prefix-sum lookups.

Changes:

• Clamp the index used in the non-selected select arm (max(ix, 1u) - 1u) across six prefix-sum lookup sites to prevent underflow on ix == 0.
• Update the Unreleased changelog with a note about the Mali device-loss fix.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
vello_shaders/shader/tile_alloc.wgsl	Clamp discarded-arm prefix-sum index to avoid - 1u underflow.
vello_shaders/shader/coarse.wgsl	Clamp discarded-arm prefix-sum indices in two prefix-sum lookups.
vello_shaders/shader/backdrop_dyn.wgsl	Clamp discarded-arm prefix-sum index in row prefix-sum lookup.
vello_shaders/shader/fine.wgsl	Clamp discarded-arm prefix-sum indices in two prefix-sum lookups.
CHANGELOG.md	Add Unreleased entry describing the Mali device-loss fix.

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