Add slang shaders for UI overlay

shaders/slang/base/uioverlay.frag.slang

@@ -0,0 +1,20 @@
+/* Copyright (c) 2025, Sascha Willems
+ *
+ * SPDX-License-Identifier: MIT
+ *
+ */
+
+[[vk::binding(0, 0)]]
+Sampler2D fontTexture;
+
+struct VSOutput
+{
+	[[vk::location(0)]]float2 UV : TEXCOORD0;
+	[[vk::location(1)]]float4 Color : COLOR0;
+};
+
+[shader("fragment")]
+float4 main(VSOutput input) : SV_TARGET
+{
+	return input.Color * fontTexture.Sample(input.UV);
+}