Make image chunk type ref struct

[Poker-sang]

I've found that most of the image block types (such as PngPhysical, WebpVp8X, etc.) are only used as intermediate variables for reading and writing images, not as members.

Maybe we can change these types to ref struct for efficiency.

[antonfirsov]

Is there any codegen advantage from ref struct-s besides being able to hold ref members or other ref struct-s?

[Poker-sang]

ref struct forces types to be used on the stack to reduce unnecessary gc and boxing

[antonfirsov]

struct locals always live on the stack, and boxing will only happen if you do something in the code that will box your struct. What ref struct adds on top of that is that it prevents such boxing at compilation type, enabling you to hold ref-s in the type (they cannot marshalled to the heap). AFAIK making all structs ref struct-s "just in case" is not a favorable practice.