Sometimes when using direct scan-out, some flickering between the
fullscreen app and the regular desktop could be seen.
This happened because we called wlr_output_attach_render and then
wlr_output_attach_buffer for direct scan-out. wlr_output_attach_render
makes the OpenGL context current but also attaches the OpenGL buffer to
the primary plane apparently (all of this happens inside
eglMakeCurrent).
This patch moves the scan-out logic outside of output_render, before
wlr_output_attach_render. This lines it up with rootston's
implementation. This also makes more sense since no rendering is
involved when using direct scan-out.
Sorry about that, I should've tested this with more clients. The new
code has been tested with mpv and a GLFW demo.