+/**
+ * typedef for fragment shader function
+ *
+ * @param context jit context
+ * @param x block start x
+ * @param y block start y
+ * @param facing is front facing
+ * @param a0 shader input a0
+ * @param dadx shader input dadx
+ * @param dady shader input dady
+ * @param color color buffer
+ * @param depth depth buffer
+ * @param mask mask of visible pixels in block
+ * @param thread_data task thread data
+ * @param stride color buffer row stride in bytes
+ */