[Banshee] Filter is applied after gamma

[leilei-]

Describe the bug
The video filter is applied after the gamma correction process, rendering the filter ineffective.

To Reproduce

• Set gamma to 1.0, see the filter work.
• Set gamma to anything higher, see the filter not work.

Expected behavior
The gamma correction applying to the filtered buffer.

Additional context
When I've first reverse-engineered the filter in 2013, I did it on a 3dfx Voodoo3 3000 and relied on gamma behavior to observe the filter results with dark lines on black backgrounds.