Fix edge case in composite function interpolation (#8613)

This fixes a bug where, for a zoom value greater than that of the highest zoom
stop, composite function interpolation would return nan. (Blocking a render
test over in #8593)

* Add failing tests for composite function edge case

The failing cases here are:
 - Should interpolate before the first stop
 - Should interpolate past the last stop

* Fix edge case in composite function interpolation

* Hold functions constant outside stop-defined domain

1 files changed, 8 insertions, 1 deletions

diff --git a/include/mbgl/style/function/composite_function.hpp b/include/mbgl/style/function/composite_function.hpp
index b31329b75e..2b4ae504ca 100644
--- a/include/mbgl/style/function/composite_function.hpp
+++ b/include/mbgl/style/function/composite_function.hpp
@@ -56,9 +56,13 @@ public:
                 assert(!s.stops.empty());
                 auto minIt = s.stops.lower_bound(zoom);
                 auto maxIt = s.stops.upper_bound(zoom);
-                if (minIt != s.stops.begin()) {
+                
+                // lower_bound yields first element >= zoom, but we want the *last*
+                // element <= zoom, so if we found a stop > zoom, back up by one.
+                if (minIt != s.stops.begin() && minIt->first > zoom) {
                     minIt--;
                 }
+                
                 return std::make_tuple(
                     Range<float> {
                         minIt == s.stops.end() ? s.stops.rbegin()->first : minIt->first,
@@ -96,6 +100,9 @@ public:
     T evaluate(float zoom, const GeometryTileFeature& feature, T finalDefaultValue) const {
         std::tuple<Range<float>, Range<InnerStops>> ranges = coveringRanges(zoom);
         Range<T> resultRange = evaluate(std::get<1>(ranges), feature, finalDefaultValue);
+        // If the covering stop range is constant, just return the output value directly.
+        if (resultRange.min == resultRange.max) return resultRange.min;
+        // Otherwise, interpolate between the two stops.
         return util::interpolate(
             resultRange.min,
             resultRange.max,