Draft: render: introduce wlr_renderer_download()

Simon Ser requested to merge emersion/wlroots:renderer-download into master

Similar to wlr_renderer_read_pixels(), but will work with render passes. While at it, pass in a region so that callers don't need to call this function multiple times (that way, renderers only need to map/unmap buffers once).

TODO: implement in renderers, use in screencopy impl

Edited by Simon Ser

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