implement texture null check flag on shaders

Sets a flag if the texture being sent to the shader is null

app/shaders/alphaover.frag

@@ -4,10 +4,23 @@ varying vec2 ove_texcoord;
 
 uniform sampler2D base_in;
 uniform sampler2D blend_in;
+uniform bool base_in_enabled;
+uniform bool blend_in_enabled;
 
 void main(void) {
   vec4 base_col = texture2D(base_in, ove_texcoord);
   vec4 blend_col = texture2D(blend_in, ove_texcoord);
+
+  if (!base_in_enabled) {
+    gl_FragColor = blend_col;
+    return;
+  }
+
+  if (!blend_in_enabled) {
+    gl_FragColor = base_col;
+    return; 
+  }
+  
   base_col *= 1.0 - blend_col.a;
   base_col += blend_col;