Possibly allow the host to avoid server Metal pipeline state if they will always blit

[bangnoise]

SyphonMetalServer always creates a SyphonServerRendererMetal with associated pipeline state. This will never be used if the host only provides blittable textures. Although we could lazily initialise it, that would cause some delay to the first frame published.

Possibly add an option to be passed to the options dictionary which takes a MTLTextureDescriptor for textures the host intends to publish, and skip SyphonServerRendererMetal creation if the textures will be blittable.

The harm in this will be somewhat mitigated by #85.