Create/Update assets for version 1.21.3

assets/minecraft/shaders/post/entity_outline_box_blur.fsh

@@ -0,0 +1,18 @@
+#version 150
+
+uniform sampler2D InSampler;
+
+in vec2 texCoord;
+in vec2 sampleStep;
+
+out vec4 fragColor;
+
+void main() {
+    vec4 blurred = vec4(0.0);
+    float radius = 2.0;
+    for (float a = -radius + 0.5; a <= radius; a += 2.0) {
+        blurred += texture(InSampler, texCoord + sampleStep * a);
+    }
+    blurred += texture(InSampler, texCoord + sampleStep * radius) / 2.0;
+    fragColor = vec4((blurred / (radius + 0.5)).rgb, blurred.a);
+}