This lets compositors check whether direct scan-out is possible. Compositors will soon be responsible for manually calling this function.
wlr_output is not well-suited to checking whether direct scan-out is happening or not. Compositors may want to use their own external swapchains, for instance.
Additionally, ab7eabac ("output: leverage wlr_output_configure_primary_swapchain()") makes it so output_basic_test() is called before the output swapchain is initialized, resulting in false positives.
Closes: #3620 (closed)
Breaking change: compositors now need to check
wlr_output_is_direct_scanout_allowed() before direct scan-out.