Implemented observer callbacks so the style knows when the source’s attribution changes and the map knows when the style’s attribution changes. Also implemented a getter for a tile source’s attribution.
Fixes#2723.
This function can be called from different threads and was depending
on a global `std::string protocol` which can do heap allocations.
The first thread to use it would initialized it which is not exactly
safe, so this patch moves it to a `char *` that is just a pointer to
the data segment, so no races.
Valgrind detected it on #6431.
```
==9874== Conditional jump or move depends on uninitialised value(s)
==9874== at 0x4C307C2: __memcmp_sse4_1 (in /home/travis/build/mapbox/mapbox-gl-native/mason_packages/linux-x86_64/valgrind/latest/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==9874== by 0x79896E: mbgl::util::mapbox::isMapboxURL(std::string const&) (in /home/travis/build/mapbox/mapbox-gl-native/build/qt-linux-x86_64/Release/mbgl-test)
==9874== by 0x6DC756: mbgl::style::TileSourceImpl::parseTileJSON(std::string const&, std::string const&, mbgl::SourceType, unsigned short) (in /home/travis/build/mapbox/mapbox-gl-native/build/qt-linux-x86_64/Release/mbgl-test)
==9874== by 0x6DDA6A: std::_Function_handler<void (mbgl::Response), mbgl::style::TileSourceImpl::loadDescription(mbgl::FileSource&)::{lambda(mbgl::Response)#1}>::_M_invoke(std::_Any_data const&, mbgl::Response&&) (in /home/travis/build/mapbox/mapbox-gl-native/build/qt-linux-x86_64/Release/mbgl-test)
==9874== by 0x50BE80: std::_Function_handler<void (), mbgl::StubFileSource::StubFileSource()::{lambda()#1}>::_M_invoke(std::_Any_data const&) (in /home/travis/build/mapbox/mapbox-gl-native/build/qt-linux-x86_64/Release/mbgl-test)
==9874== by 0x70972A5: QMetaObject::activate(QObject*, int, int, void**) (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.2.1)
==9874== by 0x70A3621: QTimer::timerEvent(QTimerEvent*) (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.2.1)
==9874== by 0x7098053: QObject::event(QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.2.1)
==9874== by 0x60CCC8B: QApplicationPrivate::notify_helper(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5.2.1)
==9874== by 0x60D1E55: QApplication::notify(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt5Widgets.so.5.2.1)
==9874== by 0x706FC2C: QCoreApplication::notifyInternal(QObject*, QEvent*) (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.2.1)
==9874== by 0x70BC1AC: QTimerInfoList::activateTimers() (in /usr/lib/x86_64-linux-gnu/libQt5Core.so.5.2.1)
==9874==
```
* [ios, macos] Allow full hex colors in layer docs
CSS hexadecimal RGB syntax can contain all the hexadecimal digits, not just the decimal digits.
* [ios, macos] Parse colors with csscolorparser
A new MGLTileSet class that wraps all available core Tileset options can be created and passed into the sources. The sources pass that tileset object along to core.
* [core, ios, android] Use `auto` value for properties with calculated defaults
* Fix render tests
* [core] Don't use depth test unless text is pitch-aligned to map.
* Bump mapbox-gl-test-suite
* TransformState pitch is already in radians
* Reduce setDepthSublayer calls
* Bump test suite
