gl-renderer: Add renderer-follows-scale support

Support render_scale factors for a downsampled renderer output buffer,
rendering into a viewport of the output's logical size rather than the
native mode size.

This also requires support from the backends to correctly allocate and
scale the buffers.

Signed-off-by: Daniel Stone <daniels@collabora.com>
3 jobs for render-scale in 1 minute and 30 seconds (queued for 2 seconds)
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