Create data transmitting class for fgviewer

[show50726]

• Add FrameGraphInfo class for transmitting data between the engine and the server
  • It implements the pimpl pattern
• Adjust DebugServer api to align with the design
  • Has an unique handle to track the views instead of hashing the view name as the key, since the view name is not guaranteed to be unique

[poweifeng]

Sorry I had one more comment that somehow got lost in the shuffle.

Your code has a lot of definitions of this form:

void func(Object copy) {
  value = std::move(copy);
}

I think typically

void func(const Object& obj) {
  value = obj; // actual copy
}

is equivalent and more common.

Or if you're transferring content ownership, then

void func(Object&& obj) {
  value = std::move(obj);
}

is the common pattern.

Any particular reason for choosing that pattern?