astcenc 6.x proposals

[solidpixel]

An (unplanned) future major release gives a chance for breaking changes to the library API. This issue captures the list of ideas to be considered.

• Sizes in the library API should be size_t - we've had a few problems with 32-bit overflows, so using size_t everywhere removes one class of problems (but not all of them).
• Users would like to be able to set an input and output image row stride in the API (Provide an option to specify the buffer row extent when decoding #533).
• Currently we compile for C++14 - we could update to something newer (C++20 has some useful bits).