change downsample generation to use RenderParams and Render client

[RussTorres]

Changing asap.materialize.render_downsample_sections.RenderSectionAtScale.downsample_specific_mipmapLevel to use custom RenderParams with renderapi.client.render_renderparameters removes the need for a temporary stack of modified tilespecs, which produce extraneous collections in the case of aborted processing.