Image textures were being packed into a single buffer for OpenCL, which limited the amount of memory available for images to the size of one buffer (usually 4gb on AMD hardware). By packing textures into multiple buffers that limit is removed, while simultaneously reducing the number of buffers that need to be passed to each kernel. Benchmarks were within 2%. Fixes T51554. Differential Revision: https://developer.blender.org/D2745