Add an experimental "no-copy" renderer

[Adlai-Holler]

Normally UIGraphicsGetImage makes a copy of the bitmap data. Since we are done with the context, we don't need to waste time and memory making a copy. We quickly rack up hundreds of megabytes of total allocations from our CGImages and even though they're reclaimed, it's inefficient. For uncached ASImageNode renders, this happens twice per draw (opportunity: once we generate the cached contents, return them to ASDK intact rather than drawing them.)

The end result is that ASGraphicsGetImageAndEndCurrentContext is virtually free, while UIGraphicsGetImageFromCurrentImageContext and CGBitmapContextCreateImage are two of the most time- and memory-intensive functions we call. It turns out that CALayer _display does basically the same thing.

Another very very big win here is that, UIGraphicsBeginImageContext calls CGContextClearRect right after creating the context, which is a huge waste of time with no benefit (hundreds of microseconds on an iPhone 6). The bitmap context's memory is already zero'd from the kernel since it uses mmap (and we use calloc which uses mmap too).

The API is virtually the same as UIGraphics functions – ASGraphicsBeginImageContextWithOptions and ASGraphicsGetImageAndEndCurrentContext.

You call ASEnableNoCopyRendering() to enable this. If you call it after rendering has started, it asserts, returns NO, and it stays turned off.

Details are in ASGraphicsContext.h

We achieve the vertical flip and HiDPI scaling by modifying the context's CTM. UIGraphics uses a private function CGContextSetBaseCTM. The internet and docs aren't don't say what makes the base CTM so special, but it seems to be working fine. One radar said it was used for shadowing but shadowing appears to to work. We'll keep an eye out.

Profiling results with ASDKgram on an iPhone 6. I launched, waited for it to settle, then flung down once (about 2 screenfuls' worth).

Total memory allocated during display node rendering went from 60MB to 42MB – a 30% reduction.

Runs 1 and 2 are with the feature on. Runs 3 and 4 are with it off. If you look at runs 3 and 4, you can see how, at both levels (the ASDisplayNode render and the ASImageNode contents render) we have 2 equally-sized allocations in the "begin" and "end" functions. In runs 1 and 2 there's just the allocation at the beginning.

Here's a bit of rendering a text node pre-experiment with the removed bits marked:

[appleguy]

This PR is historic! The only comparable improvement I recall seeing to the display system was the custom thread pool.

So the interesting thing is, I had gotten direct information from Apple that the UIGraphics API is supposed to be Copy-on-Write — specifically that the memory would only be copied if another write occurred after an image was acquired. Maybe they dropped that feature due to issues with it? No idea, but let's get rid of that ambiguity.

[nguyenhuy]

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