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

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

gpu/src/render_pass_encoder.zig

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