[msvc] Fix warnings, mainly casting to smaller types (#270)

If the checks by GitHub Actions checks are met, then it seems good to go.  Thanks for supporting FOSS4G!

src/mbgl/style/expression/assertion.cpp

@@ -64,7 +64,7 @@ ParseResult Assertion::parse(const Convertible& value, ParsingContext& ctx) {
                 return ParseResult();
             }
             if (n) {
-                N = optional<std::size_t>(*n);
+                N = static_cast<size_t>(*n);
             }
             i++;
         }