[msvc] Fix compilation errors and warnings (#141)

platform/qt/src/utf.cpp

@@ -6,7 +6,7 @@ namespace mbgl {
 namespace util {
 
 std::u16string convertUTF8ToUTF16(std::string const& utf8) {
-    auto utf16 = QString::fromUtf8(utf8.data(), utf8.length());
+    auto utf16 = QString::fromUtf8(utf8.data(), static_cast<int>(utf8.length()));
 
     // Newers Qt have QString::toStdU16String(), but this is how it is
     // implemented. Do it here to keep compatibility with older versions.