Julia 1.7 performance regression on `rawview`

Looks like it's either MappedArrays or Julia Base issue

```julia
using ImageCore, BenchmarkTools
dst = rawview(channelview(rand(RGB{N0f8}, 64, 64)));
src = copy(dst);
@btime copyto!($dst, $src);
# 1.7.0: 1.209 ms (49152 allocations: 1.69 MiB)
# 1.6.5: 55.287 μs (0 allocations: 0 bytes)
```