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==
```
SymbolLayout lives on the worker thread and contains the persistent data needed for repeated placement. SymbolBucket contains the data generated during placement, and is transferred to the main thread for rendering.
This eliminates the risky sharing of GeometryTile::buckets between the main thread and worker thread during TileWorker::redoPlacement.
While here, rationalize the names of states a SymbolLayout may be in:
* Pending: Waiting for the necessary glyphs or icons to be available.
* Prepared: The potential positions of text and icons have been determined.
* Placed: The final positions have been determined, taking into account prior layers.
In TileWorker, all SymbolLayouts are stored in a single vector. Each SymbolLayout knows what state it is in, and TileWorker can easily determine how much progress it can make toward a final result.
* [ios] #3979 wip gzip telemetry events
* [ios] expose compression from core instead of adding a new dependency
* [ios] #3979 Exposed decompress from mbgl and added a test case
* [ios] #3979 cleanup
* [ios] #3979 cleaned up test case
* [ios] #3979 cleanup
* [ios] #3979 gzip -> deflate
* correct all EXPECT_EQ(actual, expected) to EXPECT_EQ(expected, actual)
* fix getMapboxURLPathname() of URL with querystring
* add test for normalizeSourceURL of non-mapbox protocal
* Update mapbox.cpp
- puts function definitions in a namespace ... {} rather than using namespace ...;
- remove trailing whitespace
- add trailing newline
- protect SQL statements from being formatted by clang-format
