T# word1 MIN_LOD (unsigned 4.8, in levels of the whole surface) was
rejected. Sampled views now chain VkImageViewMinLodCreateInfoEXT
(VK_EXT_image_view_min_lod, enabled when the device offers the minLod
feature). Its minLod counts levels of the whole image exactly as MIN_LOD
counts levels of the whole surface, because the image holds every mip and
the view starts at BASE_LEVEL.
- A clamp at or below BASE_LEVEL cannot change which level a sample reads
and is left off; one past the view's last level is bounded by it (the
view selects that level anyway, and Vulkan requires the bound).
- A clamp that binds on a device without the feature throws.
- The texture cache already keys by the eight descriptor words, so views
with different clamps are not shared.
- Storage views address one level: a clamp at or below that level is
accepted, one above it still throws.
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