Swaylock can run out of buffers when output is resized

[mstoeckl]

When running Sway nested under wayland or x11, it is possible to dynamically resize the outputs. Most of the time, swaylock correctly redraws when this is done. However, if rendering a frame takes a long time -- which can happen when running swaylock -i large_image.png -- sometimes swaylock fails to provide a properly sized buffer for the new output size. In this case, the background will only cover a portion of the screen, revealing whatever is underneath the rest.

This behavior stems from the limited number of background buffers associated with a given struct swaylock_surface. If swaylock is made to render a new frame background at least three times in short succession, it will render and submit two buffers, but will run out of buffers on the third call to render_frame_background. This can happen if the output is resized many times in a second, so that swaylock receives a large number of layer surface configure events in one batch; in that case, it will try to render a new frame background for each.