feat: deduplicate text streams in period flattening

[valotvince]

Title said it all :)

In a multi-period stream with the same text streams over periods, we would get back duplicates text tracks when calling getTextTracks player API.

That PR should fix that behaviour.

[joeyparrish]

Looks good otherwise. Thanks!

[joeyparrish]

Minor nit: typo on "duplicates"

Larger concern: the test has no expectations on the text streams, so it would pass even without your code change. Please add a check to the end of the test to show that t3 was filtered out. Thanks!

[valotvince]

Didn't see that one 😅

[joeyparrish]

Thank you!

[shaka-bot]

Test Failure:

Chrome 85.0.4183.121 (Linux x86_64)
  DashParser Manifest sets contentType to text for embedded text mime types FAILED
    Error: Expected 1 to be 2.
        at <Jasmine>
        at _callee41$ (test/dash/dash_parser_manifest_unit.js:1056:41 <- test/dash/dash_parser_manifest_unit.js:1268:49)
        at tryCatch (node_modules/babel-polyfill/dist/polyfill.js:6900:40)
        at Generator.invoke [as _invoke] (node_modules/babel-polyfill/dist/polyfill.js:7138:22)

    TypeError: Cannot read property 'type' of undefined
        at _callee41$ (test/dash/dash_parser_manifest_unit.js:1060:36 <- test/dash/dash_parser_manifest_unit.js:1273:44)
        at tryCatch (node_modules/babel-polyfill/dist/polyfill.js:6900:40)
        at Generator.invoke [as _invoke] (node_modules/babel-polyfill/dist/polyfill.js:7138:22)
        at Generator.prototype.<computed> [as next] (node_modules/babel-polyfill/dist/polyfill.js:6952:21)
        at step (test/dash/dash_parser_manifest_unit.js:3:191)
        at test/dash/dash_parser_manifest_unit.js:3:361

Failed 1 tests, passed 1961, skipped 31

[shaka-bot]

All tests passed!