OwrImageRenderer: Limit frame rate using videorate max-rate

If we use videorate with max-rate and drop-only=true, we should be able
to apply framerate limitations without adding any latency. This also
avoids caps negotiation issues with drop-only=true, input caps as 0/1 or
just generally having higher output framerate than input.
OwrLocalMediaSource: Improve frame rate limiting

Use drop-only=true max-rate=fps to limit video frame rate with zero
latency.
OwrVideoRenderer: Remove unnecessary conversion elements

glimagesink can do colour space conversion and scaling itself. Unless
we're at the limits of performance with high framerate and high
resolution, limiting framerate is unnecessary.
OwrVideoRenderer: Make queue downstream leaky
OwrImageRenderer: Rename elements to image-renderer-*