FullscreenUI: Change default background and add Strobe

data/resources/fullscreenui/backgrounds/Default.glsl

@@ -3,7 +3,7 @@ void main()
   // Radial gradient at (0.6, 0.4), moving horizontally slowly
   float r1 = length(v_tex0 - vec2(0.6, 0.4));
   float r2 = length(v_tex0 - vec2(0.61, 0.41));
-  float r = mix(r1, r2, sin(u_time / 10.0));
-  vec3 bg_color = vec3(r * 0.33);
+  float r = mix(r1, r2, sin(u_time / 5.0));
+  vec3 bg_color = vec3(0.1, 0.1, 0.6) * (1.0f - r);
   o_col0 = vec4(bg_color, 1.0);
 }

data/resources/fullscreenui/backgrounds/Strobe.glsl

@@ -0,0 +1,10 @@
+void main()
+{
+  vec2 uv = v_tex0 * 2.0 - 1.0;
+  uv.x *= u_display_size.x / u_display_size.y;
+
+  vec2 center = vec2(cos(u_time * 0.2) * 1.5, (sin(u_time * 0.2) * 0.2) * 0.5 + 1.2);
+  float dist = length(uv - center);
+  float gradient = smoothstep(0.0, 1.0, dist);
+  o_col0 = vec4(mix(vec3(0.5, 0.5, 0.9), vec3(0.05, 0.05, 0.2), gradient), 1.0);
+}

src/core/fullscreen_ui.cpp

@@ -230,7 +230,7 @@ static void GetStandardSelectionFooterText(SmallStringBase& dest, bool back_inst
 // Backgrounds
 //////////////////////////////////////////////////////////////////////////
 
-static constexpr const char* DEFAULT_BACKGROUND_NAME = "Trails";
+static constexpr const char* DEFAULT_BACKGROUND_NAME = "Default";
 
 static bool HasBackground();
 static void LoadBackground();