vulkan: leave the alpha test reference out of pipelines that don't test alpha

FixedPipelineState stored alpha_test_ref whatever the alpha test
function. Title 1 animates the register with the test off (text fading
in and out), so every value made a new pipeline whose shaders were
lowered and translated on the GPU thread: bursts of 600-1,900 pipelines
in 10 s (up to 5 s of GPU thread) whenever text was on screen. The
reference is now kept only while the test compares with it (not Always
or Never). Driving title 1's menus: 400+ pipelines for already-known
shaders before, 0 after; one shader lowered 79 times before, 2 after.

EDEN_NX2_PIPELINE_DIFF=1 logs which fixed-state fields differ when a new
pipeline reuses known shaders.

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