FR: add expression evaluation in defines

[munrocket]

Add WIN_WIDTH / WIN_HEIGHT in prelude with ability to write expressions in defines

// usual case
#define GROUP_COUNT_X WIN_WIDTH/16
#define GROUP_COUNT_Y WIN_HEIGHT/16

// example 1
struct Data {
  foo: array<array<u32, GROUP_COUNT_Y>, GROUP_COUNT_X>
}
#storage data Data

// example 2 (this calculated automatically somehow)
#workgroup_count bar GROUP_COUNT_X GROUP_COUNT_Y 1
@compute @workgroup_size(16, 16)
fn bar(@builtin(global_invocation_id) gid: vec3u) {
  //...
}

[munrocket]

Basically it Is the same as @Builtin(num_workgroups) but in top level. Without this code become not portable or slow.

[davidar]

The preprocessor already defines SCREEN_WIDTH and SCREEN_HEIGHT https://github.com/compute-toys/wgpu-compute-toy/blob/master/src/lib.rs#L470-L471

Perhaps that could be documented better.

Evaluating constant expressions in the preprocessor would be handy, ideally we could use some library for that to avoid adding too much ad hoc complexity.