Create/Update assets for version 1.21.3

assets/minecraft/shaders/core/position.fsh

@@ -0,0 +1,16 @@
+#version 150
+
+#moj_import <minecraft:fog.glsl>
+
+uniform vec4 ColorModulator;
+uniform float FogStart;
+uniform float FogEnd;
+uniform vec4 FogColor;
+
+in float vertexDistance;
+
+out vec4 fragColor;
+
+void main() {
+    fragColor = linear_fog(ColorModulator, vertexDistance, FogStart, FogEnd, FogColor);
+}