Only schedule frame callback from on_process

draw was called in the pipewire on_process callback, but this is not
safe as libpipewire runs on_process on a separate thread.

Instead, kickstart a frame callback loop if needed and let the
Wayland-side drive drawing on the main thread.
readme: Update discuss section
Move peak_decay to input side
Reflow wl_surface_damage_buffer
Disable werror for now
Move display_dispatch closer to use
Set opaque region
Clean up shm slightly
Switch from EGL to software rendering
Swap ALSA out with PipeWire
Draw on frame callback
layer-shell: Use bottom layer
readme: Elaborate on function
Initial commit