GPU Backend #273

Adding GPU backend support to LCUI's graphics library is expensive, so I plan to make LCUI support drawing using other open source graphics libraries.

I've improved the graphics library of LCUI in recent weeks and wrote an example program to draw graphics using the Cairo graphics library.

https://github.com/lc-soft/LCUI/tree/add-cairo-example/lib/pandagl/examples/cairo

I also have plans to write an example to draw UI widgets using Cairo, but it may take a long time.

why not try the direct2d-based vuilib, it's a student project but is both pretty and performant!