alphaover: return black if no textures are enabled

Not doing this appeared to corrupt the composited buffer somehow and return a
black image if there were two empty texture connections daisychained together
on two AlphaOver instances.

app/shaders/alphaover.frag

@@ -11,6 +11,11 @@ void main(void) {
   vec4 base_col = texture2D(base_in, ove_texcoord);
   vec4 blend_col = texture2D(blend_in, ove_texcoord);
 
+  if (!base_in_enabled && !blend_in_enabled) {
+    gl_FragColor = vec4(0.0);
+    return;
+  }
+
   if (!base_in_enabled) {
     gl_FragColor = blend_col;
     return;