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

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

gpu/src/compute_pass_encoder.zig

@@ -1,6 +1 @@
-pub const ComputePassEncoder = enum(usize) {
-    _,
-
-    // TODO: verify there is a use case for nullable value of this type.
-    pub const none: ComputePassEncoder = @intToEnum(ComputePassEncoder, 0);
-};
+pub const ComputePassEncoder = *opaque {};