gpu: add AlphaMode enum

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

gpu/src/enums.zig

@@ -138,6 +138,11 @@ pub const TextureUsage = enum(u32) {
     present = 0x00000020,
 };
 
+pub const AlphaMode = enum(u32) {
+    premultiplied = 0x00000000,
+    unpremultiplied = 0x00000001,
+};
+
 test "name" {
     try std.testing.expect(std.mem.eql(u8, @tagName(Feature.timestamp_query), "timestamp_query"));
 }