Add PacketOps support for textures

Texture channel data is packed contiguously, so leverage PacketOps for gathers (and scatters) that show substantial speed-ups (particularly on the LLVM backend).

This is especially true for half-precision textures, where the absence of FP16 SIMD gather instructions results in bloated compiled kernels and a degradation in performance relative to the single-precision counterpart. PacketOps circumvents this issue by avoiding the use of gather intrinsics entirely.