Add image accumulation entry point

I was hoping to implement the image filter/reduction step as well, but that seems to be meaningfully more complex

enkou-shaders-tests/src/lib.rs

@@ -54,6 +54,9 @@ mod test {
 
     #[test]
     fn has_entry_main_camera() {
-        assert!(has_entry_point(ExecutionModel::GLCompute, "main_camera"))
+        assert!(has_entry_point(
+            ExecutionModel::GLCompute,
+            "main_image_accumulate"
+        ))
     }
 }