bug: `srcpixel` param of `DeepData::merge_deep_pixels` should be `int64_t` (breaks ABI)

[luna-y-kim]

Found during the nodiscard attribute task.
There are a couple of places that need follow-up to #2363.
The srcpixel param of DeepData::merge_deep_pixels should be widened from int to int64_t, but it breaks compatibility (fails the ABI check), so it is left as int for now.
Another int to int64_t change is included in #5241.

void merge_deep_pixels(int64_t pixel, const DeepData& src, int srcpixel);

• deepdata.h:191
• deepdata.cpp:1179