Check for ext protocol support

[sewnie]

With the toplevel protocol being supported, no NULL check was put in place for it if a compositor does not support it. Here, the check was rewritten to check for the screencast type's required protocol.

Fixes #337

[kennylevinsen]

How did you trigger this crash?

state->screencast_source_types only includes WINDOW if ext_foreign_toplevel_iamge_capture_source_manager is set, so I'm surprised if a client is asking for it.

I suspect it would be cleaner to handle this in setup_target instead anyway, to ensure we don't show options to the user that we shouldn't have:

type_mask &= ctx->state->screencast_source_types;
if (type_mask == 0) {
    logprint(ERROR, "wlroots: No supported targets specified");
    return;
}

[sewnie]

How did you trigger this crash?

Use a compositor that has ext_foreign_toplevel_image_capture_source_manager as NULL.

state->screencast_source_types only includes WINDOW if ext_foreign_toplevel_iamge_capture_source_manager is set

When I go to select the window, I can select a window.

I suspect it would be cleaner to handle this in setup_target instead anyway, to ensure we don't show options to the user that we shouldn't have:

Good point. Shall i add this fix? you wrote it, so it isn't exactly right of me to steal it :p

[kennylevinsen]

Feel free to steal it, you discovered the issue anyway. When reviewers share code snippets it's usually with the intent that you can take whatever is useful to complete the work.