silence warning in native_switch_context!

often occurs on Julia exit, and the warning doesn't tell you much
by itself

Author: jwahlstrand

src/window.jl

@@ -34,7 +34,7 @@ function ShaderAbstractions.native_switch_context!(w::GtkWindow)
     if haskey(win2glarea, w)
         ShaderAbstractions.native_switch_context!(win2glarea[w])
     else
-        @warn("Unable to find GtkGLArea in native_switch_context!")
+        @debug("Unable to find GtkGLArea in native_switch_context!")
     end
 end
 function ShaderAbstractions.native_context_alive(x::GtkWindow)