Half-pixel shift in the downscaling step

Should match chroma on most videos now, which makes the shader a little bit better.

This obviously breaks it when you have centre chroma, but hopefully this isn't a big issue.

CfL_Prediction.glsl

@@ -30,7 +30,7 @@
 //!DESC Chroma From Luma Prediction (Downscaling Luma)
 
 vec4 hook() {
-    return LUMA_texOff(0.0);
+    return LUMA_texOff(vec2(-0.5, 0.0));
 }
 
 //!HOOK CHROMA

CfL_Prediction_Lite.glsl

@@ -30,7 +30,7 @@
 //!DESC Chroma From Luma Prediction (Downscaling Luma)
 
 vec4 hook() {
-    return LUMA_texOff(0.0);
+    return LUMA_texOff(vec2(-0.5, 0.0));
 }
 
 //!HOOK CHROMA