Fixed depth in Front-to-Back DVR

In front-to-back direct volume rendering, `col` is the total amount of colour that the ray gained in front of the current value of depth. We can't do it in back-to-front DVR, so `src.a > 0.15` is still a proper approach.

Assets/Shaders/DirectVolumeRenderingShader.shader

@@ -302,7 +302,7 @@
                     src.rgb *= src.a;
                     col = (1.0f - col.a) * src + col;
 
-                    if (src.a > 0.15 && t < tDepth) {
+                    if (col.a > 0.15 && t < tDepth) {
                         tDepth = t;
                     }
 #endif