Fixed|UI|Client: Fatal error when activating splitscreen (viewgrid)

The "ui/background" material no longer exists, as the UI is being
drawn by libappfw and (at least presently) it has no ties to the
material subsystem.

Replaced the obsolete UI background drawer with a black quad.

doomsday/client/src/ui/ui_main.cpp

@@ -350,6 +350,7 @@ void UI_DrawRectEx(const Point2Raw* origin, const Size2Raw* size, int border, dd
 
 void UI_DrawDDBackground(Point2Raw const &origin, Size2Raw const &dimensions, float alpha)
 {
+    /*
     ui_color_t const *dark  = UI_Color(UIC_BG_DARK);
     ui_color_t const *light = UI_Color(UIC_BG_LIGHT);
     float const mul = 1.5f;
@@ -359,8 +360,9 @@ void UI_DrawDDBackground(Point2Raw const &origin, Size2Raw const &dimensions, fl
         App_ResourceSystem().material(de::Uri("System", Path("ui/background")))
             .prepare(UI_MaterialSpec(TSF_MONOCHROME));
     GL_BindTexture(&ms.texture(MTU_PRIMARY));
+    */
 
-    glEnable(GL_TEXTURE_2D);
+    glDisable(GL_TEXTURE_2D);
     if(alpha < 1.0)
     {
         GL_BlendMode(BM_NORMAL);
@@ -370,19 +372,20 @@ void UI_DrawDDBackground(Point2Raw const &origin, Size2Raw const &dimensions, fl
         glDisable(GL_BLEND);
     }
 
+    glColor4f(0, 0, 0, alpha);
     glBegin(GL_QUADS);
         // Top color.
-        glColor4f(dark->red * mul, dark->green * mul, dark->blue * mul, alpha);
-        glTexCoord2f(0, 0);
+        //glColor4f(dark->red * mul, dark->green * mul, dark->blue * mul, alpha);
+        //glTexCoord2f(0, 0);
         glVertex2f(origin.x, origin.y);
-        glTexCoord2f(1, 0);
+        //glTexCoord2f(1, 0);
         glVertex2f(origin.x + dimensions.width, origin.y);
 
         // Bottom color.
-        glColor4f(light->red * mul, light->green * mul, light->blue * mul, alpha);
-        glTexCoord2f(1, 1);
+        //glColor4f(light->red * mul, light->green * mul, light->blue * mul, alpha);
+        //glTexCoord2f(1, 1);
         glVertex2f(origin.x + dimensions.width, origin.y + dimensions.height);
-        glTexCoord2f(0, 1);
+        //glTexCoord2f(0, 1);
         glVertex2f(0, origin.y + dimensions.height);
     glEnd();