Broke up the iOS and macOS Bitrise scripts into several steps each for more granular progress and failure reporting and to take advantage of better error reporting in the built-in Bitrise steps. Added a step to deploy Xcode test results and (in the event of a failure) raw xcodebuild output to Bitrise.io, with a handy link from the Bitrise UI.
* Main gyp files are now standardized as platform/<platform>/platform.gyp.
* Each platform gyp file defines appropriate loop_lib and headless_lib variables.
* Each platform gyp file includes mbgl.gypi, which defines base targets which may be useful to all platforms.
* CI targets are consistent across platforms: `make $(PLATFORM) && make test-$(PLATFORM)`.
* Renamed the "linux" test app to "glfw". It's now built in OS X CI.
* Android build flakiness is fixed.
* iOS CI builds the bench and iosapp targets.
* Mesa version is now in one place.
* CI scripts use bash "strict mode" and correct error handling.
* All build output goes to the build directory.
* Removed vestigial iOS/OS X/Android Travis scripts.
Replaced appledoc usage with jazzy, which understands modern Objective-C syntax by virtue of using Clang ASTs. Nevertheless, we have to make lots of changes to our documentation syntax, which was tailored to appledocs quirks. The new syntax jives much better with what Xcode expects in terms of auto-indentation and Quick Help.
Fixes#1420.
