gpu: convert Buffer from enum(usize) to *opaque

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

gpu/src/bind_group.zig

@@ -9,7 +9,7 @@ pub const BindGroup = *opaque {};
 pub const BindGroupEntry = extern struct {
     next_in_chain: *const ChainedStruct,
     binding: u32,
-    buffer: Buffer = Buffer.none, // nullable
+    buffer: ?Buffer,
     offset: u64,
     size: u64,
     sampler: Sampler = Sampler.none, // nullable