Fix clang tidy error and crash (flutter/engine#42564)
Clang tidy checks for https://github.com/flutter/engine/pull/42399 are
failing for `formats_mtl.mm` and `context_mtl.mm`. Weirdly this happens
even though none of these files where modified in the branch.
For `context_mtl.mm` the fix is straightforward, just a missing
`std::move`.
For `formats_mtl.mm` clang-tidy actually crashes on the assignments to
`des.depthCompareFunction` and `des.depth_write_enabled`. The crash
doesn't happen if the assignment is done through a temporary variable.
I'm not sure if we should have a workaround for the crash or disable
clang-tidy on the file completely (FLUTTER_NOLINT).
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