Moltenvk support on macos/ios

Apple is deprecating OpenGL in favour of Metal on all its platforms. This means that glimagesink is going to be less relevant going forward on macOS/iOS.

This adds support for integrating MoltenVK into cerbero in order to write GPU-accelerated code using cross-platform Vulkan instead of the vendor-specific Metal.

Depends on gst-plugins-bad!284 (merged)