implement rotateBy by specifying an angle in degree

include/mbgl/map/map.hpp

@@ -77,6 +77,7 @@ class Map : private util::noncopyable {
     void scaleBy(double scale, const std::optional<ScreenCoordinate>& anchor, const AnimationOptions& animation = {});
     void pitchBy(double pitch, const AnimationOptions& animation = {});
     void rotateBy(const ScreenCoordinate& first, const ScreenCoordinate& second, const AnimationOptions& = {});
+    void rotateBy(double angleDegree, const AnimationOptions& = {});
     CameraOptions cameraForLatLngBounds(const LatLngBounds&,
                                         const EdgeInsets&,
                                         const std::optional<double>& bearing = std::nullopt,

src/mbgl/map/map.cpp

@@ -184,6 +184,12 @@ void Map::rotateBy(const ScreenCoordinate& first, const ScreenCoordinate& second
     impl->onUpdate();
 }
 
+void Map::rotateBy(double angleDegree, const AnimationOptions& animation) {
+    impl->cameraMutated = true;
+    impl->transform.rotateBy(angleDegree, animation);
+    impl->onUpdate();
+}
+
 CameraOptions Map::cameraForLatLngBounds(const LatLngBounds& bounds,
                                          const EdgeInsets& padding,
                                          const std::optional<double>& bearing,

src/mbgl/map/transform.cpp

@@ -506,6 +506,16 @@ void Transform::rotateBy(const ScreenCoordinate& first,
     easeTo(CameraOptions().withBearing(bearing), animation);
 }
 
+void Transform::rotateBy(double angleDegree, const AnimationOptions& animation) {
+    const double bearing = -util::rad2deg(state.getBearing()) - angleDegree;
+    easeTo(CameraOptions().withBearing(bearing), animation);
+}
+
+/*!
+ * \brief Transform::getBearing
+ * Bearing in radiant
+ * \return bearing in radians
+ */
 double Transform::getBearing() const {
     return state.getBearing();
 }

src/mbgl/map/transform.hpp

@@ -86,6 +86,7 @@ class Transform : private util::noncopyable {
     // Bearing
 
     void rotateBy(const ScreenCoordinate& first, const ScreenCoordinate& second, const AnimationOptions& = {});
+    void rotateBy(double angleDegree, const AnimationOptions& = {});
     double getBearing() const;
 
     // Pitch

src/mbgl/map/transform_state.hpp

@@ -310,6 +310,10 @@ class TransformState {
 
     // map position
     double x = 0, y = 0, z = 0;
+    /*!
+     * \brief bearing
+     * Bearing in radian
+     */
     double bearing = 0;
     double scale = 1;
     double fov = util::DEFAULT_FOV;