renderer/opengl: comptime assertion to verify

src/renderer/opengl/CellProgram.zig

@@ -64,6 +64,13 @@ pub const CellMode = enum(u8) {
     }
 
     pub fn isFg(self: CellMode) bool {
+        // Since we use bit tricks below, we want to ensure the enum
+        // doesn't change without us looking at this logic again.
+        comptime {
+            const info = @typeInfo(CellMode).Enum;
+            std.debug.assert(info.fields.len == 5);
+        }
+
         return @intFromEnum(self) & @intFromEnum(@as(CellMode, .fg)) != 0;
     }
 };