[core] Define max cache size for ImageManager

include/mbgl/util/constants.hpp

@@ -48,6 +48,10 @@ constexpr uint8_t DEFAULT_PREFETCH_ZOOM_DELTA = 4;
 
 constexpr uint64_t DEFAULT_MAX_CACHE_SIZE = 50 * 1024 * 1024;
 
+// Default ImageManager's cache size for images added via onStyleImageMissing API.
+// Average sprite size with 1.0 pixel ratio is ~2kB, 8kB for pixel ratio of 2.0.
+constexpr std::size_t DEFAULT_ON_DEMAND_IMAGES_CACHE_SIZE = 100 * 8192;
+
 constexpr Duration DEFAULT_TRANSITION_DURATION = Milliseconds(300);
 constexpr Seconds CLOCK_SKEW_RETRY_TIMEOUT { 30 };