Add wlr_output_impl.rollback

Most of the pending output state is not forwarded to the backend prior
to an output commit. For instance, wlr_output_set_mode just stashes the
mode without calling any wlr_output_impl function.
wlr_output_impl.commit is responsible for applying the pending mode.

However, there are exceptions to this rule. The first one is
wlr_output_attach_render. It won't go away before renderer v6 is
complete, because it needs to set the current EGL surface.

The second one is wlr_output_attach_buffer.
wlr_output_impl.attach_buffer is removed in [1].

When wlr_output_rollback is called, all pending state is supposed to be
cleared. This works for all the state except the two exceptions
mentionned above. To fix this, introduce wlr_output_impl.rollback.

Right now, the backend resets the current EGL surface. This prevents GL
commands from affecting the output after wlr_output_rollback.

This patch is required for FBO-based outputs to work properly. The
compositor might be using FBOs for its own purposes [2], having leftover
FBO state can have bad consequences.

[1]: https://github.com/swaywm/wlroots/pull/2097
[2]: https://github.com/swaywm/wlroots/pull/2063#issuecomment-597614312

types/wlr_output.c

@@ -606,6 +606,10 @@ bool wlr_output_commit(struct wlr_output *output) {
 
 void wlr_output_rollback(struct wlr_output *output) {
 	output_state_clear(&output->pending);
+
+	if (output->impl->rollback) {
+		output->impl->rollback(output);
+	}
 }
 
 void wlr_output_attach_buffer(struct wlr_output *output,