Create/Update assets for version 1.21-pre3

assets/minecraft/shaders/program/invert.fsh

@@ -0,0 +1,16 @@
+#version 150
+
+uniform sampler2D DiffuseSampler;
+
+in vec2 texCoord;
+
+uniform float InverseAmount;
+
+out vec4 fragColor;
+
+void main(){
+    vec4 diffuseColor = texture(DiffuseSampler, texCoord);
+    vec4 invertColor = 1.0 - diffuseColor;
+    vec4 outColor = mix(diffuseColor, invertColor, InverseAmount);
+    fragColor = vec4(outColor.rgb, 1.0);
+}