Fix #147782: Workbench alpha imprecision

Regression from 1c41815c4d.

The log2/exp2 round-trip can cause imprecisions, which are especially
notable with the alpha channel.
This removes the round-trip conversion for the alpha channel.

Pull Request: https://projects.blender.org/blender/blender/pulls/148218

source/blender/draw/engines/workbench/shaders/workbench_effect_smaa_frag.glsl

@@ -38,7 +38,7 @@ void main()
   }
   out_color /= taa_accumulated_weight;
   /* Exit log2 space used for Anti-aliasing. */
-  out_color = exp2(out_color) - 1.0f;
+  out_color.rgb = exp2(out_color.rgb) - 1.0f;
 
   /* Avoid float precision issue. */
   if (out_color.a > 0.999f) {

source/blender/draw/engines/workbench/shaders/workbench_effect_taa_frag.glsl

@@ -21,7 +21,7 @@ void main()
       /* Clamp infinite inputs (See #112211). */
       color = clamp(color, float4(0.0f), float4(1e10f));
       /* Use log2 space to avoid highlights creating too much aliasing. */
-      color = log2(color + 1.0f);
+      color.rgb = log2(color.rgb + 1.0f);
 
       frag_color += color * samplesWeights[i];
     }