h264_clip_a_filter_ycbcr2rgba: calling --enablePostProcessFilter 2 result in a shader error

When we call the decode with --enablePostProcessFilter 2, the decode process ends up failing with error:

```
   STDERR:
     Compilation error: 
     shaderc_error:73: error: 'outputImageRGB' : undeclared identifier
     shaderc_error:73: error: 'imageStore' : no matching overloaded function found
```