Replace RGBImagePtr by RGBImageCleanup

[vrabaud]

Although RGBImagePtr, defined using std::unique_ptr, is correct, it is
confusing because it does not free the avifRGBImage struct. Since the
avifRGBImage struct is intended to be allocated on the stack, we should
not address this by forcing the users of RGBImagePtr to allocate
avifRGBImage from the heap.

Replace RGBImagePtr by RGBImageCleanup, which is modeled after
absl::Cleanup and is a lightweight cleanup object whose destructor
calls avifRGBImageFreePixels() on the associated avifRGBImage struct.

[wantehchang]

LGTM. Thanks. I suggest some changes.

[wantehchang]

Maryla: Since I edited this PR, could you please review it? Thanks.

[wantehchang]

Vignesh: Please review. This is modeled after absl::MakeCleanup. Thanks!

[vigneshvg]

Since the new class is only to be used when the buffers are owned by the underlying pointer, can you please clarify a bit more in the commit message as to how the new class is different from the old unique_ptr wrapper?

Could one not use the existing unique_ptr wrapper the same way?

[wantehchang]

Vignesh: I added a commit message in #3048 (comment).

The existing unique_ptr wrapper (RGBImagePtr) is equivalent in functionality to the new RGBImageCleanup, so one can view this PR as only making the code less confusing. The existing unique_ptr wrapper is confusing because users expect unique_ptr to delete the struct that the pointer points to, but RGBImagePtr only frees the pixels buffer and does not free the avifRGBImage struct.