generalize allocation alignment for emscripten

Emscripten has a bug where its allocation do not align to
alignof(max_align_t). This is causing misalignment issues when
using SkParagraph in JavaScript.

Add the kAllocationAlignment to express this notion, and
us it when calculating additional memory allocation.

Change-Id: I62106d65333eca4d529b5f38a71b244b9192465e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/414396
Reviewed-by: Herb Derby <herb@google.com>
Reviewed-by: Jiulong Wang <jiulongw@vibe.us>
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Kevin Lubick <kjlubick@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed