Benchmark AV1 film grain shader and possible improvement ideas

We need to properly benchmark the AV1 shader and obtain real performance metrics to see if the use of an SSBO is actually justified.

Possible alternative:

• Replace SSBO by simple grain textures
• For the offsets, use a one-channel 8-bit integer texture and gather all four points with textureGather. That we there's some cache reuse between neighbouring pixels, in theory

This might also make it more compatible with older versions of GLSL that don't have storage shaders