[core] add setMaxZoom, setMinZoom

also change the default maxZoom to 20 to match -js.

src/mbgl/map/transform_state.cpp

@@ -168,6 +168,10 @@ double TransformState::getScale() const {
     return scale;
 }
 
+void TransformState::setMinZoom(const double minZoom) {
+    min_scale = zoomScale(minZoom);
+}
+
 double TransformState::getMinZoom() const {
     double test_scale = min_scale;
     double unused_x = x;
@@ -177,6 +181,10 @@ double TransformState::getMinZoom() const {
     return scaleZoom(test_scale);
 }
 
+void TransformState::setMaxZoom(const double maxZoom) {
+    max_scale = zoomScale(maxZoom);
+}
+
 double TransformState::getMaxZoom() const {
     return scaleZoom(max_scale);
 }