libweston: Add output parameter to weston_renderer::flush_damage()

When flush_damage() is called, the output to be repainted next is
already known. Pass it along into the renderer, which can make use
of this information:

The GL renderer can get a better idea which SHM surface textures
actually have to be updated, in case a surface can be put on a plane
on one output, but not another.

A future Vulkan renderer could record texture uploads into an output
specific command buffer.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>