If stencil is constant across the resource, then it can be treated as
if it was cleared.
Improves performance in applications which create a stencil buffer but
do not use it. Originally the same was done for depth to help some 2D
applications, but that gave mixed results so the patch was dropped.
v2: Don't do anything if a fragment job wouldn't be needed otherwise.
v3: Set stencil_value when a batch is cleared (Alyssa)
v4: Handle clears when the stencil is already known (Alyssa)
v5: Make sure shared resources are not used (Alyssa)
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16646>
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