• egl-headless + ANGLE was pretty straightforward but I wasn't satisfied with the solution because it rendered offline to a framebuffer on GPU -> framebuffer is copied to the CPU -> CPU buffer is sent to SPICE client -> SPICE client renders it by copying it back to the GPU. Although this is the only way if running a VM remotely, the movement of data back and forth from GPU to CPU is unnecessary.
• On Linux, SPICE uses dmabuf to pass a pointer to GPU data so no copying was needed. On macOS we don't have dmabuf, but we have IOSurface which has a similar mechanism. ANGLE also supports rendering to IOSurface. So we added IOSurface rendering to QEMU's spice-display.
• In order to be compatible with SPICE libraries (without having to modify the APIs), we wrap IOSurfaceID into a file descriptor by passing it through a pipe. On macOS, we have to use kIOSurfaceIsGlobal to pass IOSurfaceID across process boundaries. On iOS, everything is in one process so that's not needed. A better solution would use mach ports or XPC but it would be a lot of work to integrate that into QEMU.

Closes #161