Interleaving textures with primitives was solving a problem that does not
exist: within a layer, order is already undefined because freeing an
instance swap-removes it, and layering is what layers are for. So the run
batching is gone.
A layer is now `LayerDraws`: a `Primitives` and a texture `InstanceList`
side by side, with `updated` covering both. `Primitives` holds only
primitives again -- its instance list plus the group-1 data those
instances read -- and `InstanceList` is the shared push/free/apply_free
the two lists have in common rather than a second copy of it.
`PrimitiveHandle` names which list with `InstanceKind`, and
`PrimitiveChange` carries the same, since the two index independently.
The handle no longer carries a group-1 index at all: a primitive
instance already records where its entry is.
The renderer gives each layer a second instance buffer and draws its
textures one at a time after the instanced draw, each binding its own
group 2.
Review fixes alongside: `GlyphAtlas::allocate` returns the `PageUpload`
it reserved instead of a bare tuple; a page or image region uploads
through a new `write_region`, which passes the row stride to
`write_texture` rather than copying the rectangle out first.
Verified by replaying taps into the `tabs` example: three images added
and one deleted leaves two drawn with two live texture slots, and the
masked text-edit tab still clips, with the images freed on tab switch.
iris
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