Investigate memory usage of backends

[nicoburns]

With the winit example in this repo (and the default 800x600 logical size (with 2x scale factor)) get:

• 61792 bytes using a new null backend (which doesn't do any rendering at all and has a memory usage of 1 byte)
• 85728 bytes using the vello_cpu+pixels backend
• 95920 bytes using the vello_cpu+softbuffer backend
• 86544 bytes using the vello backend

In all cases (except the null backend) memory usage jumps up quite a bit if the window is resized bigger. To ~100mb for vello and ~120mb for vello_cpu+pixels with a fullscreen window on my MacBook Pro 14".

[karmakarmeghdip]

I was making an application with Xilem which also uses Vello, and memory usage is quite high, there as well. This is definitely an upstream issue