IGraphicsSkia::GetLayerBitmapData() crashes in iOS AUv3

[olilarkin]

seems SkSurface::readPixels() fails in app extensions

[AlexHarker]

unfortunately, I'm also seeing issues with text rendering when in app extensions - this occurs when the call to flush() happens under skia (the drop shadow can be done without SkSurface ::readPixels() as shown here:

https://github.com/iPlug2/iPlug2/tree/graphics/skia-dropshadow

which might ultimately be much more efficient on METAL anyway (haven't tested speed). We should look into the restrictions on skia when in app extension more generally though and if there might be a solution.