gpu: make requestDevice callback-based, add waitForDevice helper

Previously we were attempting to turn WebGPU async functions, which are
exposed by `webgpu.h` as callbacks, into Zig async functions.

This in practice turns out harder than expected. For example, `Buffer.mapAsync`
/ `wgpuBufferMapAsync` cannot easily be exposed as a Zig async function for a few
reasons:

1. The callback is merely guaranteed to be called once the buffer's content is ready to
   be accessed via `wgpuBufferGetMappedRange` - but there is no strict guarantee about
   when that is. It could be 1-3 frames later, in theory, I believe.
2. The non-deterministic timing means that one would wish to poll "has the async function
   returned?" but this isn't trivial without our own scheduler.
3. Zig has a fair amount of async rework in the future that is coming, and I imagine it
   will be one of the later things that is fully supported by the WebAssembly backend
   (but I am speculating) - so it seems wise to punt on this until later.

Instead, we are now retaining async functions as callback-based ones, with a helper in
this case to wait for the callback to be invoked. For `wgpuBufferMapAsync` we will just
have the callback approach.

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

gpu/examples/sample_utils.zig

@@ -92,7 +92,7 @@ pub fn setup(allocator: std.mem.Allocator) !Setup {
         props.driver_description,
     });
 
-    const device = switch (nosuspend backend_adapter.requestDevice(&.{})) {
+    const device = switch (backend_adapter.waitForDevice(&.{})) {
         .device => |v| v,
         .err => |err| {
             std.debug.print("failed to get device: error={} {s}\n", .{ err.code, err.message });