Updates nanovg and some code to adapt the changes.

Updates the nanovg repository and updates some code to adapt the changes.

This commit also creates a new "nanovg_hook.cc" file for loading nanovg implementation once to avoid "duplicate symbols" or "undefined symbols" errors. All other modules should include only the "nanovg_hook.h" header file to use nanovg features.

Signed-off-by: Olli Wang <olliwang@ollix.com>
Changes the API of retrieving system directories.

The API has been changed to match the style of iOS SDK for flexibility.

A new MOUI_APPLE constant identifier is also added to simplify the code of (MOUI_IOS || MOUI_MAC).