[FEATURE] Add Vulkan/Metal rendering support

Currently, libprojectM only supports rendering using OpenGL 3 and GLES 2. Since OpenGL is generally considered a deprecated API and Vulkan/Metal has a better API to perform multi-threaded rendering and shader compilation, it will be a good step forward to add support for these next-gen rendering APIs.

Any API-specific initialization calls, e.g. providing the required Vulkan structs, should be implemented in a specific header, e.g. libprojectM/renderer_vulkan.h. Adding support for multiple rendering backends will also require a major refactoring of the library, abstracting rendering-related calls. projectM only renders a few primitives (triangle fans/strips, line strips and points) and uses vertex and fragment shader programs, so there's not much to implement.

Line rendering should be improved beforehand, as covered by issue #682.

Vulkan support should be introduced together with the EGL migration covered by issue #681.

Start by reviewing the existing OpenGL 3 and GLES 2 rendering paths, then read the proposed libprojectM/renderer_vulkan.h interface and the related issues #681 and #682. Done means adding Vulkan and Metal rendering support while accommodating the EGL migration and the prerequisite line-rendering improvements.