Add a libFuzzer target for libktx and a Linux CI job to run it

[armatti]

tests/fuzz is a CMake project of its own that adds lib/ as a sub-project, like tests/use-ktx, and builds ktx_read with Clang, libFuzzer instrumentation and ASan + UBSan. ktx_read_fuzzer loads each input as an application loads an untrusted file: create the texture, load the image data, transcode Basis Universal data and read all of it. Headers declaring more than 64 MiB of data are skipped before the data is loaded.

The new Fuzz job in linux.yml builds it, fuzzes for 10 minutes from the small KTX and KTX2 files in tests/resources and uploads any finding. It is separate from the ASan job because libFuzzer needs Clang and its own instrumented build.

• A scheduled run with a persistent corpus (actions/cache, or ClusterFuzzLite's batch mode) would find deeper bugs than a fresh 10-minute run per PR.
• Applying to OSS-Fuzz would give continuous fuzzing on Google's infrastructure; libktx isn't there yet.

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