[spv-in] Support `OpImageGather` and `OpImageDrefGather`

[trsh]

Currently it throws invalidInstruction

[teoxoy]

for reference:
https://github.com/gfx-rs/naga/blob/01fbdea21f6537c0db95e2a2a284e61598574f2c/src/front/spv/image.rs#L611
initial implementation done in gfx-rs/naga#1596