[spv-in] support OpImage, OpImageFetch, OpLogicalOr, OpLogicalAnd, relational functions, and fix OpUndef

FragDepth is written to in a fragment shader, not read.

Fix wrong result when casting to bool
Add tests

Also, fix the Typifier's handling of logical operations.
See https://github.com/gfx-rs/naga/pull/404 for v0.3 branch.

* [spv-out] Add missing builtins

* [spv-out] remove a non-existent integer version of mix

Add support for more math functions

Change ParseError to be public
Change globals to default to private instead of throwing error