Create/Update assets for version 1.21.4-pre3

assets/minecraft/shaders/post/blur.vsh

@@ -0,0 +1,21 @@
+#version 150
+
+in vec4 Position;
+
+uniform mat4 ProjMat;
+uniform vec2 InSize;
+uniform vec2 OutSize;
+uniform vec2 BlurDir;
+
+out vec2 texCoord;
+out vec2 sampleStep;
+
+void main() {
+    vec4 outPos = ProjMat * vec4(Position.xy, 0.0, 1.0);
+    gl_Position = vec4(outPos.xy, 0.2, 1.0);
+
+    vec2 oneTexel = 1.0 / InSize;
+    sampleStep = oneTexel * BlurDir;
+
+    texCoord = Position.xy / OutSize;
+}