perf: avoid too large buffer allocations

[hannomargelo]

I noticed that displaying an png was taking a significant amount of memory.

After some profiling I found too issues that cause too much memory allocations:

1. The rows_frame and rows_buffer are png_bytep arrays. C++ will already allocate the elements to have a size of png_bytep. Us multiplying the height with the sizeof(png_bytep) is actually not needed.
   • (However, I think sizeof(png_bytep) returns 1 anyway. However, for correctness I added this change.
2. The main improvement is to only pass height * width as frame byte size to the ApngFrame:
   • Before it was size = height * row_bytes where row_bytes is basically width * channels (4)
   • But the ApngFrames internal pixel array is already a uint32_t. Meaning one value already includes the precomputed value for all channels (see saveFrame).
   • Thus the ApngFrames internal pixel array is oversized by 4x. By changing it to width * height its size is correct

We had an extreme case where the animation (and rest of the app) before took ~1.7GB:

With this change we were able to bring it down to ~700mb, a 2.3x improvement:

Related to:

• 占用内存太大 #108
• too much memory usage #74

Important

This performance improvement was sponsored by Discord 🌟

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[hannomargelo]

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