gpu: make Device an enum with methods

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

gpu/src/Device.zig

@@ -1,6 +1,10 @@
-ptr: *anyopaque,
+pub const Device = enum(usize) {
+    _,
 
-pub const LostReason = enum(u32) {
-    undef = 0x00000000,
-    destroyed = 0x00000001,
+    pub const none: Device = @intToEnum(Device, 0);
+
+    pub const LostReason = enum(u32) {
+        undef = 0x00000000,
+        destroyed = 0x00000001,
+    };
 };