[Impeller] Generate mipmaps with render passes on devices with broken blit generation - #190264

…cts (flutter#190186)

Fixes bdero/flutter_scene#241. Related to flutter#161283.

Adreno drivers corrupt blit-generated mip chains. The workaround for
that rewrote every Vulkan sampler to `MipFilter::kBase`, so nothing in
the process could be sampled past its base mip level. But the corruption
belongs to the texture whose levels were generated, not to the sampler,
so textures with hand-uploaded mip chains (prefiltered environment maps,
for one) lost their mip filtering for no reason. Flutter GPU is hit
hardest, since it exposes no mipmap generation at all and every mip
chain it binds was uploaded by the app.

The workaround now lives with the texture. On affected drivers a mipped
sampled texture also gets an image view covering only the base level,
and `TextureVK::GetSampledImageView` hands that view out for textures
those drivers mipped. Generated chains stay unreadable past the base
level, every other texture samples as requested, and the sampler
library, `SamplerDescriptor`, and `lib/gpu` no longer carry any of it.

A texture with `mip_count > 1` whose levels were never written is no
longer masked on these devices, which matches its behavior on every
other platform. flutter#190264 fixes the generation itself and will delete what
is left here.

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