Not sure I agree there. If Gallium were present it would do filtering just fine; you would have to write your filters in TGSI rather than OpenCL but if you're running natively over Gallium then you're already using TGSI.
A bigger issue is that non-Linux platforms will typically already have a fair amount of video filtering and processing happening in the playback stack (eg DXVA). That gets into difficult tradeoffs; Gallium3D is potentially portable across OSes but OpenCL *will* be ported across OSes so you're likely to be trading off better portability (OpenCL) against better integration and efficiency (Gallium3D or something else OS-specific).
I'm glad I don't have to choose