[spv-out] Avoid generating duplicate OpTypeImage instructions.

Fixes #1305.

Ensure that two `back::spv::LocalType::Image` values are sure to be equal (and
hash equal) whenever they would generate the same `OpTypeImage` instruction, so
that the usual duplicate removal via `Writer::lookup_type` works. Accomplish
this by changing the contents of `LocalType::Image` to more closely match the
operands of `OpTypeImage` instructions.

Previously, `LocalType::Image` included a `ImageClass::Storage::access` value,
which did not affect the `OpTypeImage` instruction generated. If two
`LocalType::Image` values differered only in their `access`, then they would get
separate entries in `Writer::lookup_type`, two identical `OpTypeImage`
instructions would be generated, and SPIR-V validation would fail.