fix_telecined_fades: make tff/bff handling simpler/saner

Reflect the actual field placement in the property names.

vsdeinterlace/combing.py

@@ -55,16 +55,15 @@ def fix_telecined_fades(
     props_clip = core.akarin.PropExpr(
         [f.work_clip, fields[::2], fields[1::2]], lambda: {  # type: ignore[misc]
             f'f{t}Avg{i}': f'{c}.P{i}Average {color} -'  # type: ignore[has-type]
-            for t, c in ['ty', 'bz']
+            for t, c in zip('tb' if tff.is_tff else 'bt', 'yz')
             for i, color in zip(f.norm_planes, f.norm_seq(colors))
         }
     )
 
     fix = norm_expr(
-        props_clip, 'Y 2 % x.f{t1}Avg{i} x.f{t2}Avg{i} ? AVG! '
+        props_clip, 'Y 2 % x.fbAvg{i} x.ftAvg{i} ? AVG! '
         'AVG@ 0 = x x {color} - x.ftAvg{i} x.fbAvg{i} + 2 / AVG@ / * ? {color} +',
         planes, i=f.norm_planes, color=colors, force_akarin=func,
-        t1='b' if tff.is_tff else 't', t2='t' if tff.is_tff else 'b'
     )
 
     return f.return_clip(fix)