Create/Update assets for version 1.21

assets/minecraft/shaders/core/blit_screen.vsh

@@ -0,0 +1,11 @@
+#version 150
+
+in vec3 Position;
+
+out vec2 texCoord;
+
+void main() {
+    vec2 screenPos = Position.xy * 2.0 - 1.0;
+    gl_Position = vec4(screenPos.x, screenPos.y, 1.0, 1.0);
+    texCoord = Position.xy;
+}