[FR] Add libplacebo support

[drtaru]

Hello, this project is awesome, but I would love to have libplacebo support!

[wader]

Hey, did some research but i'm no libplacebo expert. As i understand it will require either vulkan or opengl? at least vulkan seems to not be possible to build statically atm KhronosGroup/Vulkan-Loader#1538 haven't looked closer at opengl yet.