[core] RenderTile does not expose Tile

RenderTile and Tile will be split when ochestration movces to a separate thread.

This PR improves encapsulation of Tiles management: now performed within TilePyramid.

src/mbgl/tile/geometry_tile.cpp

@@ -168,12 +168,12 @@ void GeometryTile::getImages(ImageRequestPair pair) {
     imageManager.getImages(*this, std::move(pair));
 }
 
-const optional<ImagePosition> GeometryTile::getPattern(const std::string& pattern) {
+const optional<ImagePosition> GeometryTile::getPattern(const std::string& pattern) const {
     auto it = iconAtlas.patternPositions.find(pattern);
     if (it !=  iconAtlas.patternPositions.end()) {
         return it->second;
     }
-    return {};
+    return nullopt;
 }
 
 void GeometryTile::upload(gfx::UploadPass& uploadPass) {