Commit 3f5a090b1b added a reference to
`STANDARD_MATERIAL_FLAGS_BASE_COLOR_UV_BIT`, a nonexistent identifier,
in the alpha discard portion of the prepass shader. Moreover, the logic
didn't make sense to me. I think the code was trying to choose between
the two UV sets depending on which is present, so I made it do that.
I noticed this when trying Bistro with #13277. I'm not sure why this
issue didn't manifest itself before, but it's clearly a bug, so here's a
fix. We should probably merge this before 0.14.
846757cb38
