egl_help_set_text and egl_help_render were both accessing bmp->help from
different threads. This creates a race condition in which if the help text
is quickly toggled on and off, it stays on.
This has been fixed with an atomic exchange.
This should prevent the looking-glass-client window from having an alpha
channel. On Wayland, the alpha channel is used to compose the window onto
the desktop, so the wallpaper would bleed through unless set to complete
opaque.
We worked around this by using constant alpha for rendering, but it was
not sustainable. Instead, we should just ask for 24-bit context.
