Minimum dimensions are 64x64, which is not really needed

[Former user]

On this line: https://github.com/videolan/x265/blob/master/source/common/param.cpp#L1481

it is checked that the dimensions are at least maxCUSize (so at least 64x64), which is unnecessarily restrictive. If the dimensions are at least minCUSize (8x8), things still work. This is relevant for encoding small images as HEIF.