df generation: single allocation with calloc

df generation: single allocation with calloc

The dfStorage DFData allocation can never fit in its stack space: 5px padding on each side always implies at least a 10x10 DFData allocation, but the stack space only fits 64 DFData.

So we've always been spilling to the heap.

If we're going to spill to the heap, we might as well allocate/free all our temporary memory in one block, and since we want it zeroed, might as well calloc.

So in practice this replaces 1-2 malloc, 1-2 free, and 2 bzeros with 1 calloc and 1 free.

BUG=skia:4729
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Committed: https://skia.googlesource.com/skia/+/12204d90337656542a42fa0fcccb7bec13af0cce

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Review URL: https://codereview.chromium.org/1544983004