libplacebo: refactor vout to be backend-agnostic, add support for OpenGL

There's no reason that the vulkan/display.c, which effectively does not depend on vulkan specifics (except for context creation), should be vulkan-exclusive. libplacebo supports OpenGL and (soon) Direct3D as well, so we this vout should be backend-agnostic and support any API supported by libplacebo.

This series deletes the old vulkan vout module and adds a new libplacebo vout, which is functionally identical but moves context creation to a separate family of modules, with implementations for Vulkan and OpenGL.

TODO:

• Squash !65 (82c20fcd) into previous commits
• Decide what to do about the OpenGL / GLES module(s)