Fix raster masking bug (#2798)

src/mbgl/gfx/index_vector.hpp

@@ -123,6 +123,7 @@ public:
         static_assert(sizeof...(args) % groupSize == 0, "wrong buffer element count");
         assert(!released);
         util::ignore({(v.emplace_back(std::forward<Args>(args)), 0)...});
+        dirty = true;
     }
 };
 

src/mbgl/renderer/buckets/raster_bucket.cpp

@@ -113,6 +113,9 @@ void RasterBucket::setMask(TileMask&& mask_) {
         segment.vertexLength += vertexLength;
         segment.indexLength += 6;
     }
+
+    vertices.updateModified();
+    indices.updateModified();
 }
 
 bool RasterBucket::hasData() const {