Maintain rotation when fitting to bounds

Also fit to the rotated bounds. A little more verbose than necessary due to <http://stackoverflow.com/a/2357688/4585461>.

ref mapbox/mapbox-gl-js#1338

src/mbgl/map/map.cpp

@@ -147,31 +147,39 @@ void Map::setLatLngZoom(LatLng latLng, double zoom, Duration duration) {
 }
 
 void Map::fitBounds(LatLngBounds bounds, EdgeInsets padding, Duration duration) {
-    // Zoom level calculation below assumes no rotation.
-    setBearing(0);
+    // Calculate the bounds of the possibly rotated `bounds` parameter with respect to the viewport.
+    vec2<> nwPixel = pixelForLatLng({bounds.ne.latitude, bounds.sw.longitude});
+    vec2<> swPixel = pixelForLatLng(bounds.sw);
+    vec2<> sePixel = pixelForLatLng({bounds.sw.latitude, bounds.ne.longitude});
+    vec2<> nePixel = pixelForLatLng(bounds.ne);
+    vec2<> visualBounds = {
+        (std::max(std::max(nwPixel.x, swPixel.x),
+                  std::max(sePixel.x, nePixel.x)) -
+         std::min(std::min(nwPixel.x, swPixel.x),
+                  std::min(sePixel.x, nePixel.x))),
+        (std::max(std::max(nwPixel.y, swPixel.y),
+                  std::max(sePixel.y, nePixel.y)) -
+         std::min(std::min(nwPixel.y, swPixel.y),
+                  std::min(sePixel.y, nePixel.y))),
+    };
 
     // Calculate the zoom level.
-    vec2<double> nePixel = pixelForLatLng(bounds.ne);
-    vec2<double> swPixel = pixelForLatLng(bounds.sw);
-    vec2<double> size = nePixel - swPixel;
-    double scaleX = (getWidth() - padding.left - padding.right) / size.x;
-    double scaleY = (getHeight() - padding.top - padding.bottom) / size.y;
-    double minZoom = getMinZoom();
-    double maxZoom = getMaxZoom();
+    double scaleX = (getWidth() - padding.left - padding.right) / visualBounds.x;
+    double scaleY = (getHeight() - padding.top - padding.bottom) / visualBounds.y;
     double minScale = std::fmin(scaleX, scaleY);
     double zoom = std::log2(getScale() * minScale);
-    zoom = std::fmax(std::fmin(zoom, maxZoom), minZoom);
+    zoom = std::fmax(std::fmin(zoom, getMaxZoom()), getMinZoom());
 
     // Calculate the center point of a virtual bounds that is extended in all directions by padding.
-    vec2<double> paddedNEPixel = {
+    vec2<> paddedNEPixel = {
         nePixel.x + padding.right / minScale,
         nePixel.y + padding.top / minScale,
     };
-    vec2<double> paddedSWPixel = {
+    vec2<> paddedSWPixel = {
         swPixel.x - padding.left / minScale,
         swPixel.y - padding.bottom / minScale,
     };
-    vec2<double> centerPixel = (paddedNEPixel + paddedSWPixel) * 0.5;
+    vec2<> centerPixel = (paddedNEPixel + paddedSWPixel) * 0.5;
     LatLng centerLatLng = latLngForPixel(centerPixel);
 
     setLatLngZoom(centerLatLng, zoom, duration);