Create/Update assets for version 1.21.5-pre3

assets/minecraft/shaders/core/gui.fsh

@@ -0,0 +1,15 @@
+#version 150
+
+in vec4 vertexColor;
+
+uniform vec4 ColorModulator;
+
+out vec4 fragColor;
+
+void main() {
+    vec4 color = vertexColor;
+    if (color.a == 0.0) {
+        discard;
+    }
+    fragColor = color * ColorModulator;
+}