Do not use Arena while rendering

[squid233]

In the tutorial, Arena is used in drawFrame and recordCommandBuffer. However, using Arena will allocate and free memory on heap in each frame, which results in bad performance.

A memory stack should be introduced.

[ice1000]

Chuigda told me the creation of an arena is very, very fast, so it won't matter too much

[chuigda]

Chuigda told me the creation of an arena is very, very fast, so it won't matter too much

Not very precise convey. According to my measurement:

• In pure benchmarking, depending on how you use it, performance of Arena.ofConfined + Arena.allocate is 1/3 - 1/5 of (overrungl) MemoryStack.pushLocal + MemoryStack.allocate
• In actual example application, time spent on native memory allocation is < 2% of whole frame time

Though performance of Arena does not matter in our example code yet, user may want to optimize native memory allocation under certain circumstances (for example, when doing more complicated command buffer recording). A MemoryStack is indeed needed in ffm-plus module.